chore: migrate Android example to AGP 9

[marandaneto]

💡 Motivation and Context

Migrate the Android example app to AGP 9.2.1 and Gradle 9.4.1, while aligning the example Android project structure with the latest Flutter Android template.

Fixes #390.

💚 How did you test it?

• cd example/android && ./gradlew help --no-daemon --console=plain
• cd example/android && ./gradlew :app:compileDebugKotlin --no-daemon --console=plain
• cd example/android && ./gradlew :app:assembleRelease --no-daemon --console=plain
• cd example/android && ./gradlew build --dry-run --no-daemon --console=plain
• cd example && flutter build apk --release
• Verified posthog_flutter/android Gradle configuration with AGP 8.11.1 using Gradle 8.14 and an injected Flutter compileSdkVersion property

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file

[marandaneto]

Fixed the release Kotlin target mismatch in 79fdc59 by configuring all KotlinCompile tasks to use JVM 1.8 for both the AGP 9 example app and the plugin's AGP 7/8 path. Also added the missing example ProGuard rules file required by AGP 9 release minification. Verified :app:assembleRelease now passes.

[marandaneto]

Pushed f94acba to explicitly keep built-in Kotlin enabled for the migrated example app. Flutter 3.44's migrator otherwise appends android.builtInKotlin=false, which opts the example back out of the migration. I verified release assembly with built-in Kotlin enabled, including with a stale ignored GeneratedPluginRegistrant.java present locally.

[marandaneto]

Fixed the Flutter APK lookup failure in 8f7acfb. The Gradle 9 layout.buildDirectory migration was resolving to example/android/build instead of example/build; changed it to ../../build so Flutter finds build/app/outputs/flutter-apk/app-release.apk again. Verified with cd example && flutter build apk --release.