[Xamarin.Android.Build.Tasks] remove the code that XA0121 deprecated

Context: #3654
Context: https://docs.microsoft.com/en-us/xamarin/android/release-notes/10/10.1#xa0121-deprecation-warning-for-old-xamarinandroidsupport-library-versions

In VS 16.4 / Xamarin.Android 10.1, we deprecated
`<GetAdditionalResourcesFromAssemblies/>`.

This PR removes the behavior completely for 16.5, since it uses SHA1.
We aim to remove all SHA1 and MD5 usage for 16.5.

NuGet packages I've seen that no longer work:

* Android Support Libraries, 25.x and older
* Google Play Services older than 44.x

I have still been able to use an old Xamarin.Forms (2.3.4) in
combination with newer support libraries.

~~ XA0121 is now an error ~~

We give a build error now in `<FilterAssemblies/>` if we encounter the
unsupported attributes. This target runs very early in the build, so
we have to do it at this point.

~~ JcwGen-Tests ~~

These tests were using these attributes such as:

    [assembly: Android.IncludeAndroidResourcesFromAttribute ("./",
        SourceUrl="file:///JavaLib.zip")]
    [assembly: Java.Interop.JavaLibraryReference ("classes.jar",
        SourceUrl="file:///JavaLib.zip")]
    [assembly: Android.NativeLibraryReference ("arm64-v8a/libsimple.so",
        SourceUrl="file:///NativeLib.zip", Version="native-lib-1")]

Instead, I switched to use `@(LibraryProjectZip)` for the `.aar` file
and `@(EmbeddedNativeLibrary)` for the `.so` files.

~~ Designer Tests ~~

The designer tests build the MyDriving app:

https://github.com/Azure-Samples/MyDriving

Which uses old NuGet packages that now fail, and is now in read-only
mode on Github. I suspect we may just need to remove that test?

    error XA0121: Assembly 'Xamarin.Android.Support.CustomTabs' is using
        '[assembly: Java.Interop.JavaLibraryReferenceAttribute]', which is no longer supported.
        Use a newer version of this NuGet package or notify the library author.

Author: jonathanpeppers

Documentation/guides/messages/xa0121.md

@@ -1,21 +1,21 @@
 ---
-title: Xamarin.Android warning XA0121
-description: XA0121 warning code
-ms.date: 09/19/2019
+title: Xamarin.Android error XA0121
+description: XA0121 error code
+ms.date: 10/30/2019
 ---
-# Xamarin.Android warning XA0121
+# Xamarin.Android error XA0121
 
 ## Issue
 
 The behavior implemented in the
-`<GetAdditionalResourcesFromAssemblies/>` MSBuild task is now
-deprecated.
+`<GetAdditionalResourcesFromAssemblies/>` MSBuild task is no longer
+supported.
 
 This MSBuild task is a precursor to [Xamarin.Build.Download][xbd] that
 enables downloading of Android packages from the internet.
 
 Libraries using any of the following custom assembly-level attributes
-will encounter this warning:
+will encounter this error:
 
 * `IncludeAndroidResourcesFromAttribute`
 * `NativeLibraryReferenceAttribute`
@@ -26,9 +26,8 @@ will encounter this warning:
 The [Xamarin Support Libraries][supportlibs], can be simply updated to
 a newer version on NuGet.
 
-Library authors will need to remove usage of these deprecated
-attributes. Their functionality will be removed in a future version of
-Xamarin.Android.
+Library authors will need to remove usage of these attributes. Their
+functionality was removed in Xamarin.Android 10.2.
 
 [xbd]: https://www.nuget.org/packages/Xamarin.Build.Download
 [supportlibs]: https://github.com/xamarin/AndroidSupportComponents

src/Mono.Android/Android/IncludeAndroidResourcesFromAttribute.cs

@@ -3,7 +3,7 @@
 namespace Android {
 
 	[AttributeUsage (AttributeTargets.Assembly, AllowMultiple = true)]
-	[Obsolete ("This attribute is deprecated and will be removed in a future release.")]
+	[Obsolete ("This attribute is not longer supported.", error: true)]
 	public class IncludeAndroidResourcesFromAttribute : ReferenceFilesAttribute
 	{
 		public IncludeAndroidResourcesFromAttribute (string path)

src/Mono.Android/Android/NativeLibraryReferenceAttribute.cs

@@ -3,7 +3,7 @@
 namespace Android {
 
 	[AttributeUsage (AttributeTargets.Assembly, AllowMultiple = true)]
-	[Obsolete ("This attribute is deprecated and will be removed in a future release.")]
+	[Obsolete ("This attribute is not longer supported.", error: true)]
 	sealed public class NativeLibraryReferenceAttribute : ReferenceFilesAttribute
 	{
 		public NativeLibraryReferenceAttribute (string filename)

src/Mono.Android/Android/ReferenceFilesAttribute.cs

@@ -1,7 +1,7 @@
 using System;
 
 namespace Android {
-	[Obsolete ("This attribute is deprecated and will be removed in a future release.")]
+	[Obsolete ("This attribute is not longer supported.", error: true)]
 	public abstract class ReferenceFilesAttribute : Attribute
 	{
 		internal ReferenceFilesAttribute () {}

src/Mono.Android/Java.Interop/JavaLibraryReferenceAttribute.cs

@@ -3,7 +3,7 @@
 namespace Java.Interop {
 
 	[AttributeUsage (AttributeTargets.Assembly, AllowMultiple = true)]
-	[Obsolete ("This attribute is deprecated and will be removed in a future release.")]
+	[Obsolete ("This attribute is not longer supported.", error: true)]
 	public class JavaLibraryReferenceAttribute : Android.ReferenceFilesAttribute
 	{
 		public JavaLibraryReferenceAttribute (string filename)

src/Xamarin.Android.Build.Tasks/MSBuild/Xamarin/Android/Xamarin.Android.Aapt.targets

@@ -48,7 +48,6 @@ Copyright (C) 2019  Microsoft Corporation. All rights reserved.
       ExtraArgs="$(AndroidResgenExtraArgs)"
       ToolPath="$(AaptToolPath)"
       ToolExe="$(AaptToolExe)"
-      AdditionalAndroidResourcePaths="@(_AdditonalAndroidResourceCachePaths)"
       ApiLevel="$(_AndroidTargetSdkVersion)"
       AndroidUseLatestPlatformSdk="$(AndroidUseLatestPlatformSdk)"
       ResourceNameCaseMap="$(_AndroidResourceNameCaseMap)"
@@ -81,7 +80,6 @@ Copyright (C) 2019  Microsoft Corporation. All rights reserved.
      AssetDirectory="$(MonoAndroidAssetsDirIntermediate)"
      ToolPath="$(AaptToolPath)"
      ToolExe="$(AaptToolExe)"
-     AdditionalAndroidResourcePaths="@(_AdditonalAndroidResourceCachePaths)"
      ApiLevel="$(_AndroidTargetSdkVersion)"
      AndroidUseLatestPlatformSdk="$(AndroidUseLatestPlatformSdk)"
      ResourceNameCaseMap="$(_AndroidResourceNameCaseMap)"

src/Xamarin.Android.Build.Tasks/MSBuild/Xamarin/Android/Xamarin.Android.Aapt2.targets

@@ -50,7 +50,7 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
     Condition=" '$(_AndroidUseAapt2)' == 'True' "
   >
   <CollectNonEmptyDirectories
-      Directories="@(LibraryResourceDirectories);@(_AdditonalAndroidResourceCachePaths->'%(Identity)\res')"
+      Directories="@(LibraryResourceDirectories)"
       LibraryProjectIntermediatePath="$(_AndroidLibrayProjectIntermediatePath)"
       StampDirectory="$(_AndroidStampDirectory)">
     <Output TaskParameter="Output" ItemName="_LibraryResourceDirectories" />

src/Xamarin.Android.Build.Tasks/MSBuild/Xamarin/Android/Xamarin.Android.Designer.targets

@@ -19,12 +19,6 @@ Copyright (C) 2016 Xamarin. All rights reserved.
 <Target Name="GetExtraLibraryLocationsForDesigner"
     DependsOnTargets="_SetupDesignTimeBuildForCompile"
     Returns="ExtraJarLocation;ExtraResourceLocation">
-  <ReadAdditionalResourcesFromAssemblyCache
-      Condition="Exists('$(_AndroidResourcePathsCache)')"
-      CacheFile="$(_AndroidResourcePathsCache)">
-    <Output TaskParameter="AdditionalAndroidResourcePaths"  ItemName="_AdditionalAndroidResourcePaths" />
-    <Output TaskParameter="AdditionalJavaLibraryReferences" ItemName="_AdditionalJavaLibraryReferences" />
-  </ReadAdditionalResourcesFromAssemblyCache>
   <ReadLibraryProjectImportsCache
       Condition="Exists('$(_AndroidLibraryProjectImportsCache)')"
       CacheFile="$(_AndroidLibraryProjectImportsCache)">
@@ -41,9 +35,6 @@ Copyright (C) 2016 Xamarin. All rights reserved.
     <ExtraJarLocation Include="@(AndroidJavaLibrary->'%(FullPath)')">
       <Source>AndroidJavaLibrary</Source>
     </ExtraJarLocation>
-    <ExtraResourceLocation Include="@(_AdditionalAndroidResourcePaths->'%(Identity)\res')">
-      <Source>AssemblyCache</Source>
-    </ExtraResourceLocation>
     <ExtraResourceLocation Include="@(_LibraryResourceDirectories)">
       <Source>LibraryImport</Source>
     </ExtraResourceLocation>

src/Xamarin.Android.Build.Tasks/Tasks/FilterAssemblies.cs

@@ -43,7 +43,7 @@ public override bool RunTask ()
 				using (var pe = new PEReader (File.OpenRead (assemblyItem.ItemSpec))) {
 					var reader = pe.GetMetadataReader ();
 					var assemblyDefinition = reader.GetAssemblyDefinition ();
-					var targetFrameworkIdentifier = assemblyDefinition.GetTargetFrameworkIdentifier (reader);
+					var targetFrameworkIdentifier = GetTargetFrameworkIdentifier (assemblyDefinition, reader);
 					if (string.Compare (targetFrameworkIdentifier, TargetFrameworkIdentifier, StringComparison.OrdinalIgnoreCase) == 0) {
 						output.Add (assemblyItem);
 						continue;
@@ -71,6 +71,40 @@ public override bool RunTask ()
 			return !Log.HasLoggedErrors;
 		}
 
+		string GetTargetFrameworkIdentifier (AssemblyDefinition assembly, MetadataReader reader)
+		{
+			string targetFrameworkIdentifier = null;
+			foreach (var handle in assembly.GetCustomAttributes ()) {
+				var attribute = reader.GetCustomAttribute (handle);
+				var name = reader.GetCustomAttributeFullName (attribute);
+				switch (name) {
+					case "System.Runtime.Versioning.TargetFrameworkAttribute":
+						var arguments = attribute.GetCustomAttributeArguments ();
+						foreach (var p in arguments.FixedArguments) {
+							// Of the form "MonoAndroid,Version=v8.1"
+							var value = p.Value?.ToString ();
+							if (!string.IsNullOrEmpty (value)) {
+								int commaIndex = value.IndexOf (",", StringComparison.Ordinal);
+								if (commaIndex != -1) {
+									targetFrameworkIdentifier = value.Substring (0, commaIndex);
+									break;
+								}
+							}
+						}
+						break;
+					case "Android.IncludeAndroidResourcesFromAttribute":
+					case "Android.NativeLibraryReferenceAttribute":
+					case "Java.Interop.JavaLibraryReferenceAttribute":
+						Log.LogCodedError ("XA0121",
+							$"Assembly '{reader.GetString (assembly.Name)}' is using '[assembly: {name}]', which is no longer supported. Use a newer version of this NuGet package or notify the library author.");
+						break;
+					default:
+						break;
+				}
+			}
+			return targetFrameworkIdentifier;
+		}
+
 		bool HasReference (MetadataReader reader)
 		{
 			foreach (var handle in reader.AssemblyReferences) {