[pigeon] Fix native interop (JNI) calls in minified Android release builds - #13151

Flutter minifies Android release builds with R8, which removes or renames classes that it doesn't see being used. JNI looks up classes and members by name at runtime, from Dart code that R8 can't see. As a result, native interop calls that work in debug builds fail in release builds with ClassNotFoundException or NoSuchMethodError.

This PR:

• Kotlin generator: When useJni is set, adds @Keep to every generated class that JNI reaches by name:

  • data classes, including sealed classes;
  • enums and their companion objects;
  • the FlutterApi interface and its Registrar;
  • the error class.

  The HostApi interface and Registrar already had @Keep. The test plugin and example app are regenerated.

• Docs: Documents keep rules for six Kotlin standard library classes. package:jni and the generated Dart code look these up by name to call and implement suspend (@async) methods. Library classes can't be annotated, so these rules are needed until package:jni ships them ([jnigen] Suspend functions fail in minified release builds: keep the Kotlin classes jni looks up by name dart-lang/native#3732).

  • Updated native_interop_guide.md, native_interop_migration_guide.md and the migration skill.
  • Each spot has a TODO linking that issue.

• CI: Adds an android_kotlin_native_interop_release_tests suite to the Linux_android custom_package_tests shard. It runs the native interop integration tests in a minified release build.

  • flutter test and flutter drive don't support release builds on Android, so the suite uses flutter run --release and reads the test summary from its output.
  • The test plugin adds the six Kotlin rules in consumer-rules.pro, with a TODO to delete them once [jnigen] Suspend functions fail in minified release builds: keep the Kotlin classes jni looks up by name dart-lang/native#3732 is fixed.
  • It deliberately has no rule for its own package, so a missing @Keep on any generated class still fails the suite.

Results for the native interop suite (339 tests) in a minified release build on an arm64 emulator:

• Before: 163 tests didn't run. R8 had removed the FlutterApi interface, so registering the FlutterApi threw ClassNotFoundException during test setup.
• Without the Kotlin rules: every failure is a call through a suspend function. Each one fails with NoSuchMethodError, because R8 renames kotlin.coroutines.Continuation.

Fixes flutter/flutter#193880
Temporary fix for dart-lang/native#3732

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• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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