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

[gabrieldonadel]

Problem

Android Gradle Plugin 9 ships built-in Kotlin support and enables it by default, so
AGP registers the kotlin extension itself. When a library also applies kotlin-android
explicitly, the two collide and configuration fails before anything compiles. AGP
words it two ways, both the same problem:

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

> The 'kotlin-android' plugin is no longer required for Kotlin support since AGP 9.0.

The apply is unconditional in this file, so on an AGP 9 project it cannot be built
at all. There is no consumer-side workaround short of patching the file — setting
android.builtInKotlin=false project-wide just to build one dependency is not a
reasonable ask, and that escape hatch is removed in AGP 10.

Change

Apply the plugin only when nothing has registered the kotlin extension yet:

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

Files changed:

• android/build.gradle

Why this shape

Earlier revisions of this PR derived the answer from the AGP version and the
android.builtInKotlin property. Asking for the extension directly is better on
three counts:

• It tests the condition that actually fails. The collision is "something already
  registered kotlin", so that is what the guard checks. No AGP version table to keep
  in sync.
• It cannot be fooled. android.builtInKotlin is a global switch, but built-in
  Kotlin can also be enabled per module with the com.android.built-in-kotlin plugin,
  so the global value can disagree with the module. Reading
  com.android.Version.ANDROID_GRADLE_PLUGIN_VERSION has its own trap: it resolves
  against the buildscript classpath, which is not always the AGP that ends up running.
• It covers AGP 10 for free. The android.builtInKotlin opt-out is removed there,
  and this guard needs no special case for it.

AGP	android.builtInKotlin	kotlin extension	explicit apply
8.x	unset or false	absent	yes (unchanged)
9.x	unset or true	registered by AGP	no
9.x	false	absent	yes
10+	n/a (removed)	registered by AGP	no

The guard sits after apply plugin: 'com.android.library' in every file it touches,
so AGP has already registered its extensions by the time it runs. I checked that
ordering per file rather than assuming it.

What I verified, and what I did not

• Verified end to end on a real Expo SDK 58 / React Native 0.87 project with AGP
  9.2.1 and Gradle 9.4.1: :app:assembleDebug succeeds both with
  -Pandroid.newDsl=true -Pandroid.builtInKotlin=true and with both flags off. That
  run used the earlier version-based guard; the extension guard in this revision is
  the shape now used across the rest of this sweep and already merged in several of
  those repos.
• Syntax-checked this file with Groovy's Phases.CONVERSION.
• Not run: this repo's own CI or example app. A CI run is the real confirmation
  and I could not do that from outside.

Found while sweeping 157 popular React Native libraries for AGP 9 new-DSL compatibility.
34 failed with the new DSL enabled, and 29 of those failed on exactly this — it is the
most common blocker by a wide margin.

This guard shape was suggested by the RevenueCat maintainers on
RevenueCat/react-native-purchases#1934,
who had already hit the same issue in their Capacitor and Flutter SDKs. I have
since standardised on it across this sweep.

[gabrieldonadel]

Pushed a revision that changes how the guard decides, not what it does.

Before, it derived the answer from the AGP version and the android.builtInKotlin
property. It now asks the question directly:

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

Three reasons this is the better shape:

• It tests the condition that actually fails, so there is no AGP version table to keep
  in sync.
• It cannot be fooled. android.builtInKotlin is global, but built-in Kotlin can also
  be switched on per module with com.android.built-in-kotlin, and
  com.android.Version.ANDROID_GRADLE_PLUGIN_VERSION resolves against the buildscript
  classpath, which is not always the AGP that ends up running.
• It covers AGP 10, where the android.builtInKotlin opt-out is removed, with no
  special case.

This shape was suggested by the RevenueCat maintainers on
RevenueCat/react-native-purchases#1934
and is what the rest of this sweep now uses. Behaviour on AGP 8 and older is unchanged.
Same files as before; title and description updated to match. Sorry for the extra
notification.