fix(android): skip explicit Kotlin plugin when AGP registers the kotlin extension (#787)

## 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 module, so on an AGP 9 project this
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:

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

Files changed:

- `android/build.gradle`

This tests the condition that actually fails, so there is no AGP version
table to
keep in sync, and it covers AGP 10 — where the `android.builtInKotlin`
opt-out is
removed — without a special case.

| 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.
- Confirmed both branches actually execute rather than one path always
winning: with
the flags off, `compileDebugKotlin` runs from the explicitly applied
plugin; with
  them on the build completes without it.
- Every changed file passes a Groovy `Phases.CONVERSION` syntax check.
- **Not run:** this repo's own CI or example app.

## Where this came from

A sweep of 500 popular React Native libraries against the AGP 9
defaults. 152 failed
with the new DSL enabled, and **144 of those failed on exactly this
collision** — by
far the most common blocker. Affects `react-native-enriched`,
`react-native-enriched-html` here.

The same guard shape was accepted in

[RevenueCat/react-native-purchases#1934](RevenueCat/react-native-purchases#1934),
at that maintainer's suggestion.

---------

Co-authored-by: Krystian Sienkiewicz <krystian.sienkiewicz@swmansion.com>

Author: gabrieldonadel

android/build.gradle

@@ -20,7 +20,13 @@ buildscript {
 
 
 apply plugin: "com.android.library"
-apply plugin: "kotlin-android"
+// AGP 9 ships built-in Kotlin support and registers the `kotlin` extension
+// itself. Applying the Kotlin plugin on top of it fails configuration with
+// "Cannot add extension with name 'kotlin'". Only apply it when nothing has
+// registered that extension yet.
+if (project.extensions.findByName('kotlin') == null) {
+	apply plugin: "kotlin-android"
+}
 
 apply plugin: "com.facebook.react"