build(android): support AGP 9 built-in Kotlin

[Tinnci]

Draft status

This is an AGP 9 built-in Kotlin migration draft. It removes explicit kotlin-android application from Android plugin Gradle files that already use Kotlin DSL/build configuration compatible with built-in Kotlin.

Do not treat this as merge-ready unless this repository is ready to rely on AGP 9 built-in Kotlin. If AGP 8 compatibility is still required, this should be revised to the maintainer-preferred compatibility shape instead of removing the Kotlin Gradle plugin.

Scope

• local_auth_android
• shared_preferences_android

No Dart APIs, Android manifests, SDK versions, or runtime code are changed.

Verification

• rg "kotlin-android|org.jetbrains.kotlin.android|kotlinOptions" on touched Android Gradle files returns no matches.
• git diff --check
• Consuming app validation with AGP 9: flutter pub get, flutter build apk --debug, ./gradlew -q :app:checkKotlinGradlePluginConfigurationErrors, flutter analyze --no-fatal-infos.

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