fix(android): skip explicit Kotlin plugin when AGP registers the kotlin extension

[huextrat]

Problem

Android Gradle Plugin 9 ships with built-in Kotlin support and registers the kotlin extension itself. When this library also applies kotlin-android unconditionally, Gradle configuration fails before compilation:

> Failed to apply plugin 'kotlin-android'.
   > Cannot add extension with name 'kotlin', as there is an extension already registered with that name.

Consumers should not need to disable AGP's built-in Kotlin support project-wide just to build this dependency, especially because that opt-out is removed in AGP 10.

Change

Apply kotlin-android only when no kotlin extension has already been registered:

if (project.extensions.findByName('kotlin') == null) {
  apply plugin: "kotlin-android"
}

The guard runs immediately after com.android.library, so AGP has already had a chance to register its extension.

AGP	Built-in Kotlin	kotlin extension	Explicit plugin apply
8.x	unavailable/off	absent	yes (unchanged)
9.x	enabled	registered by AGP	no
9.x	disabled	absent	yes
10+	enabled	registered by AGP	no

Verification

• ./gradlew :react-native-rate-app:compileDebugKotlin with AGP 9.2.1 and Gradle 9.3.1
• Same compilation with -Pandroid.newDsl=false -Pandroid.builtInKotlin=false
• yarn lint
• yarn test --maxWorkers=2 --coverage --no-watchman (22 tests, 100% coverage)
• yarn prepare

Adapted from huextrat/react-native-screenshot-aware#613.

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟢	Statements	100%	55/55
🟢	Branches	100%	28/28
🟢	Functions	100%	9/9
🟢	Lines	100%	55/55

Test suite run success

22 tests passing in 2 suites.

Report generated by 🧪jest coverage report action from 9580078

[huextrat]

🎉 This PR is included in version 2.1.3 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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