[in_app_purchase] Clarify billing dependency handling for Android in example README

[khosbilegt]

Clarify misleading billing setup instructions in the Android example for in_app_purchase's README.md (or rather lack of clarification).

Following the instructions of Google Play Billing documentation setup and adding the dependency in build.gradle causes Play Store connectivity issues, and must not be added.

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[gemini-code-assist]

Code Review

This pull request clarifies the Android billing dependency setup in the example's README to prevent users from manually adding a conflicting dependency, which is a valuable addition. The corresponding changelog entry has a minor typo and could be more concise. I've provided a suggestion to address this.

[khosbilegt]

Had to debug this for a couple hours and found the fix at https://stackoverflow.com/a/70589769/19938854.

LLMs couldn't figure it out. Thought adding it to the README, having it up on pub.dev and having it get indexed would be useful for others.

[bparrishMines]

LGTM

Although it's typically implied that plugins handle native dependencies because the purpose of the plugin is to provide access to the native library, I'm not opposed to adding additional documentation if multiple people may be running into this issue as shown by the stackoverflow issue.

@stuartmorgan-g for secondary review

[stuartmorgan-g]

Although it's typically implied that plugins handle native dependencies because the purpose of the plugin is to provide access to the native library, I'm not opposed to adding additional documentation if multiple people may be running into this issue as shown by the stackoverflow issue.

I don't think the vote counts on the SO issue suggest that this is a widespread problem, but I think the argument there that the setup docs that plugin clients need to follow include the gradle step is a reasonable one for adding some kind of note.

[stuartmorgan-g]

I think this can all be collapsed to a short bullet point under step 4 (which is about the same file):

• Unless you are also calling Google Play Billing directly from your app's native code, you do not need to add the com.android.billingclient dependency to your gradle file. If you do have a direct dependency, make sure that you are using the same version as in_app_purchase is using internally.

[stuartmorgan-g]

We should not show code that people should not add; an unfortunate number of people will copy and paste things without reading the context carefully, so including this line could increase issues.

[stuartmorgan-g]

should not be manually added when using this plugin

This isn't fully accurate; if an application uses the billing client directly for any reason, it should be managing its dependency.

[stuartmorgan-g]

"may" is pretty vague here; the actual problem would arise from using an incompatible version.

[stuartmorgan-g]

Google Play Billing Overview setup guide

It's not clear to me what exactly this is referencing; neither the page nor the section with the code below is called this, but its capitalized like it's a page title.