Update proguard-rules.pro

[Neelansh-ns]

Connection with issue(s)

Resolve issue:

ERROR: Missing classes detected while running R8. Please add the missing classes or apply additional keep rules that are generated in /Users/manishpatel/Documents/gen3/iv-pro-mobile/build/flutter_inappwebview_android/outputs/mapping/release/missing_rules.txt.
ERROR: R8: Missing class android.window.BackEvent (referenced from: void io.flutter.embedding.android.FlutterActivity.startBackGesture(android.window.BackEvent) and 3 other contexts)

Connected to:

The latest master channel in flutter gives the above error.

Testing and Review Notes

Screenshots or Videos

To Do

• double check the original issue to confirm it is fully satisfied
• add testing notes and screenshots in PR description to help guide reviewers
• request the "UX" team perform a design review (if/when applicable)

[chillbrodev]

Hello, I was wondering if this could get merged in shortly, this is holding up using the newest Android Gradle