Fixes https://github.com/flutter/flutter/issues/89683.

The changes to `getFlutterView` in `FlutterDeviceScreenshot` are the fix that was required, everything else was done to get tests running (such as re-generating some lockfiles and modifying the android manifest).

The code was all currently not unit tested, and there were no other easy examples to base these java unit tests off in flutter/flutter, so let me know if this approach to testing is wrong.