Migrate workout-logger/android to AGP 9.0+ for built-in Kotlin / new DSL support

[coderabbitai]

Summary

Follow-up to #59: migrate workout-logger/android to Android Gradle Plugin (AGP) 9.0+ to actually activate the android.builtInKotlin and android.newDsl gradle.properties flags that were added automatically by the Flutter migrator in #59. On the current AGP version (8.11.1), these flags are inert placeholders because Flutter's Gradle plugin only enables built-in Kotlin support when AGP major version >= 9.

Background

• workout-logger/android/gradle.properties sets android.builtInKotlin=true and android.newDsl=true, but Flutter's Gradle plugin (FlutterPluginUtils.isBuiltInKotlinEnabled) explicitly forces this to false when AGP major version < 9.
• The project currently pins com.android.application and org.jetbrains.kotlin.android to 8.11.1 / 2.2.20 in workout-logger/android/settings.gradle.kts.
• Official AGP compileSdk compatibility tables list API level 37 (already targeted in Feat/android 17 #59) as requiring minimum AGP 9.1.1, not just 9.0.1.

Required changes

1. workout-logger/android/settings.gradle.kts

   • Bump com.android.application plugin version to 9.1.1+ (not just 9.0.1, due to compileSdk 37 compatibility requirements).
   • Remove the org.jetbrains.kotlin.android plugin declaration entirely — AGP 9's new DSL is not compatible with the standalone Kotlin Android plugin; built-in Kotlin support handles this instead.

2. workout-logger/android/gradle/wrapper/gradle-wrapper.properties

   • Bump Gradle distribution to >= 9.1.0 (AGP 9.0's minimum Gradle requirement).

3. workout-logger/android/app/build.gradle.kts

   • Rewrite the ABI-specific versionCodeOverride logic, currently implemented via the legacy android.applicationVariants.configureEach { ... } API (casting output to ApkVariantOutputImpl), using the new androidComponents.onVariants(...) variant API. This is the biggest non-trivial code change required.
   • Migrate the kotlinOptions { jvmTarget = ... } block to the built-in-Kotlin equivalent DSL (e.g., kotlin { compilerOptions { jvmTarget = ... } }), verifying against Flutter's built-in Kotlin migration docs at the time of the upgrade.

4. Compatibility verification (risk items, not pure version bumps)

   • Confirm Flutter's flutter-gradle-plugin (pinned via Flutter SDK 3.44.4+ per Feat/android 17 #59) has stable-channel support for AGP major version 9 before committing to the bump.
   • Re-verify the F-Droid signing-block-stripping logic in app/build.gradle.kts (the comment about signingConfig needing to stay on one line) still holds under the new DSL/variant API.

Affected files

• workout-logger/android/settings.gradle.kts
• workout-logger/android/gradle/wrapper/gradle-wrapper.properties
• workout-logger/android/app/build.gradle.kts
• workout-logger/android/gradle.properties (flags already in place from Feat/android 17 #59, will become active)

Acceptance criteria

• AGP bumped to 9.1.1+ in settings.gradle.kts, org.jetbrains.kotlin.android plugin declaration removed.
• Gradle wrapper bumped to >= 9.1.0.
• android.applicationVariants.configureEach ABI versionCode logic migrated to androidComponents.onVariants(...).
• kotlinOptions.jvmTarget migrated to the built-in-Kotlin equivalent.
• Flutter Gradle plugin / Flutter SDK version confirmed compatible with AGP 9 on stable channel.
• F-Droid build (signing block stripping) verified to still work correctly.
• CI (.github/workflows/release.yml, .github/workflows/test.yml) confirmed to require no changes (JDK 17 already set via actions/setup-java@v4).

References

• PR: Feat/android 17 #59
• Discussion: Feat/android 17 #59 (comment) (see AGP 9 migration discussion thread)
• Requested by: @Devasy