refactor(auth,android): migrate native implementation to Kotlin

[SelaseKay]

Description

Migrates the firebase_auth Android implementation from Java to Kotlin while preserving the existing Pigeon HostApi and EventChannel behavior.

• HostApi implementation is now Kotlin (FlutterFirebaseAuthPlugin)
• Switches Pigeon codegen from javaOut to kotlinOut
• Updates the Android Gradle configuration to compile Kotlin sources across supported AGP versions
• Bumps the auth example Kotlin Gradle plugin to 2.3.0 so it can compile against firebase-auth 24.2.0 Kotlin metadata

This PR is stacked on #18581 (Swift). pigeons/messages.dart is shared, so the two migrations cannot land independently against main without a conflict. After the Swift PR merges, retarget this PR to main.

The Android example debug APK compiled successfully. Auth integration tests were not run.

Related Issues

None

Checklist

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• All existing and new tests are passing.
• I updated/added relevant documentation (doc comments with ///).
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Breaking Change

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• Yes, this is a breaking change.
• No, this is not a breaking change.

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