fix(android): skip explicit Kotlin plugin when AGP provides built-in Kotlin

[gabrieldonadel]

Problem

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

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

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

Keep the explicit apply, but skip it when AGP is already providing Kotlin:

def shouldApplyKotlinPlugin() {
    def agpMajor = com.android.Version.ANDROID_GRADLE_PLUGIN_VERSION.tokenize('.')[0].toInteger()
    if (agpMajor <= 8) {
        return true
    }
    def propertyVal = providers.gradleProperty("android.builtInKotlin").orNull
    def builtInKotlinEnabled = propertyVal != null ? propertyVal.toBoolean() : true
    return !builtInKotlinEnabled
}

if (shouldApplyKotlinPlugin()) {
    apply plugin: 'kotlin-android'
}

Files changed:

• android/build.gradle

Why the condition has this shape

Both halves are load-bearing:

• AGP 8 and older always apply. Built-in Kotlin only exists from AGP 9.0. On older
  AGP nothing else applies the Kotlin plugin, so skipping would break the build in the
  opposite direction.
• Absent is treated as enabled. android.builtInKotlin defaults to on in AGP 9, so
  only an explicit false means "AGP is not providing Kotlin, apply it yourself". This
  mirrors AGP's own default.

AGP	android.builtInKotlin	explicit apply
8.x	unset or false	yes (unchanged)
9.x	unset	no — AGP provides it
9.x	true	no — AGP provides it
9.x	false	yes — consumer opted out

com.android.Version.ANDROID_GRADLE_PLUGIN_VERSION requires no new plugin resolution,
and the apply already happens after com.android.library, so AGP is on the classpath.

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. Same
  guard, both configurations green.
• Syntax-checked this file with Groovy's Phases.CONVERSION.
• Not run: this repo's own CI or example app. The AGP <= 8 path is unchanged by
  construction, since shouldApplyKotlinPlugin() short-circuits to true there — but 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.