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

[hiroshihorie]

Summary

Migrates the Android plugin to AGP 9's built-in Kotlin while keeping older toolchains building (same pattern as flutter-webrtc/flutter-webrtc#2075):

• Apply the Kotlin Gradle Plugin only when built-in Kotlin is inactive — AGP < 9, or AGP 9 with android.builtInKotlin=false (the configuration Flutter currently ships by default while the ecosystem migrates). When AGP 9's built-in Kotlin is active it registers the kotlin extension itself and rejects KGP, so applying it is skipped.
• Set the JVM target through the kotlin { compilerOptions {} } DSL when the extension supports it (KGP 1.9+ / AGP 9 built-in Kotlin), falling back to the legacy kotlinOptions DSL for apps still on KGP 1.8.x.
• Bump the standalone buildscript fallback KGP to 2.1.0 so it is self-consistent.
• Add a changeset entry for the generated release notes.

Context

AGP 9 uses built-in Kotlin support and rejects Android plugins that still apply KGP directly. This follows the Flutter compatibility migration path instead of raising the minimum supported toolchain.

Verification

• Example app builds (flutter build apk --debug) on the current stable toolchain (AGP 8.x + modern KGP path).
• The AGP 9 built-in path mirrors the reviewed and merged flutter-webrtc implementation.

[Copilot]

Pull request overview

Updates the Flutter plugin’s Android Gradle configuration to be compatible with Android Gradle Plugin (AGP) 9’s built-in Kotlin support, avoiding the Kotlin Gradle Plugin application path that AGP 9 rejects, while keeping older AGP setups working.

Changes:

• Conditionally applies kotlin-android only when the detected AGP major version is < 9.
• Switches Kotlin JVM target configuration from android.kotlinOptions to the kotlin { compilerOptions { ... } } block.
• Adds a .changes/ entry for release-note generation.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
android/build.gradle	Applies Kotlin plugin conditionally based on AGP major version and updates Kotlin compiler options configuration.
.changes/agp9-built-in-kotlin	Adds a patch-level “fixed” changeset entry for AGP 9 Kotlin compatibility.

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