[Bug]: AGP 9 incompatibility — kotlin-android plugin must be removed

## Description

The `wakelock_plus` plugin applies the `kotlin-android` plugin in its `android/build.gradle`. Starting with AGP 9.0.0, built-in Kotlin is the default and applying the `kotlin-android` plugin causes a build failure:

```
Failed to apply plugin 'kotlin-android'.
> ⛔ Failed to apply plugin 'org.jetbrains.kotlin.android'
The 'org.jetbrains.kotlin.android' plugin is no longer required for Kotlin support since AGP 9.0.
Solution: Remove the 'org.jetbrains.kotlin.android' plugin from this project's build file.
```

## Steps to reproduce

1. Create or migrate a Flutter project to AGP 9.0.0 following https://docs.flutter.dev/release/breaking-changes/migrate-to-agp-9
2. Add `wakelock_plus` as a dependency
3. Run `flutter build apk --debug`
4. Build fails with the error above

## Current workaround

Set `android.builtInKotlin=false` in `gradle.properties` and re-add the `kotlin-android` plugin to the app's `build.gradle`. This is a temporary escape hatch that will be removed before AGP 10.0.

## Expected fix

Remove `id("kotlin-android")` from the plugin's `android/build.gradle` — AGP 9's built-in Kotlin handles Kotlin compilation without a separate plugin.

## Environment

- wakelock_plus: 1.4.0
- Flutter: 3.38.9
- Dart: 3.10.8
- AGP: 9.0.0
- Gradle: 9.3.1

## Related issues

- Flutter tracking: https://github.com/flutter/flutter/issues/181383
- Flutter migration guide: https://docs.flutter.dev/release/breaking-changes/migrate-to-agp-9
- Similar issue on plus_plugins: https://github.com/fluttercommunity/plus_plugins/issues/3745