chore(android): migrate app to Built-in Kotlin (drop standalone Kotlin Gradle Plugin)

[Zeroupper]

Problem

The Android app still applies the standalone Kotlin Gradle Plugin (KGP), which Flutter is deprecating. Building logs:

WARNING: Your Android app project applies the Kotlin Gradle Plugin, which will cause build failures in future versions of Flutter. Please migrate your app to Built-in Kotlin.

android/app/build.gradle still has:

• id "kotlin-android" (line 3)
• a kotlinOptions { ... } block (line 40)
• main.java.srcDirs += 'src/main/kotlin' (line 45)

Migration guide: https://docs.flutter.dev/release/breaking-changes/migrate-to-built-in-kotlin/for-app-developers

Current versions (post-#598 — already upgraded)

• AGP 9.0.1, Gradle 9.1.0, Kotlin 2.2.20, Flutter 3.44.6
• So this is not a version bump — it's the plugin-application migration only.

Also flagged: plugins that apply KGP

Flutter warns these dependencies apply KGP and could block future builds until each ships a Built-in-Kotlin-compatible release:

audio_streamer, camera_android_camerax, device_info_plus, flutter_activity_recognition, flutter_background, flutter_timezone, flutter_web_auth_2, health, light, location, network_info_plus, oidc_android, open_settings_plus, package_info_plus, pedometer, polar

We can only migrate the app project ourselves; for the plugin warnings we depend on upstream releases — track and bump as compatible versions land, and report/patch any that lag.

Acceptance criteria

• android/app/build.gradle migrated to Built-in Kotlin per the guide (no standalone kotlin-android apply)
• App builds on Android in release mode with no KGP deprecation warning for the app project
• Audit the listed plugins; bump those with Built-in-Kotlin-compatible releases, note the blockers for the rest

Related

• Gradle and Kotlin version upgrade is needed #598 — Gradle/Kotlin/AGP version upgrade (merged; this is the follow-up plugin migration)