[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.