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

[453zhangqiang-star]

What

Migrate the Android build so it supports AGP 9 built-in Kotlin, while keeping the legacy
Kotlin Gradle Plugin (KGP) path working for older AGP versions.

Why

AGP 9 enables built-in Kotlin and no longer allows library subprojects to apply the
Kotlin Gradle Plugin. Flutter documents this migration and warns about plugins that
still apply KGP; future Flutter versions will fail to build them:

• https://docs.flutter.dev/release/breaking-changes/migrate-to-built-in-kotlin/for-plugin-authors
• [many] Flutter maintained plugins should support AGP 9.0 flutter/flutter#181383

The same pattern has already been merged upstream in flutter_timezone 5.1.1,
wakelock_plus 1.7.0+, and sign_in_with_apple 8.2.0.

How

• Apply KGP only when built-in Kotlin is not in effect (agpMajor < 9 || android.builtInKotlin != true).
• Use pluginManager.apply(...) instead of apply plugin:, so Flutter's static KGP
  detection (which pattern-matches build file text) does not flag the plugin.
• Keep kotlinOptions on the legacy path; use kotlin.compilerOptions when built-in
  Kotlin is active.
• The minimum-Kotlin version check now runs only on the legacy path; with built-in Kotlin the check is skipped (the host no longer applies KGP).

No Dart or native API changes.

Testing

Build-configuration only; no behavior change on the legacy path.
I have not run this repo's example app build yet — happy to run it on request, or you can
trigger CI. Please tell me if you prefer a different structure and I will adjust.