fix(android): Avoid KGP warning on AGP 8.x with built-in Kotlin

[spydon]

Problem

Follow-up to #115. That change gated the Kotlin Gradle Plugin (KGP) behind an AGP version check:

if (agpMajor < 9) {
    apply plugin: 'kotlin-android'
}

The reasoning was sound, but Flutter's tooling does not detect KGP usage by evaluating this condition at runtime. In FlutterPluginUtils.detectApplyingKotlinGradlePlugin, Flutter scans each plugin's build.gradle text with a regex (it does this deliberately, because evaluating plugin state during configuration causes Gradle lifecycle ordering issues). The Groovy regex matches the literal apply plugin: 'kotlin-android' line anchored at line start, regardless of the surrounding if.

As a result, consumers on Flutter's built-in Kotlin (Flutter 3.44+) with AGP 8.x still see:

WARNING: Your app uses the following plugins that apply Kotlin Gradle Plugin (KGP): gamepads_android

The AGP version was the wrong signal: the warning depends on the text present in the build script, not on which branch executes.

Fixes #119

Change

Apply KGP via project.pluginManager.apply('org.jetbrains.kotlin.android') instead of apply plugin: 'kotlin-android'. This applies the exact same plugin, so Kotlin sources still compile on older Flutter / AGP < 9 (which have no built-in Kotlin), but the form is not matched by Flutter's regex, so gamepads_android is no longer flagged. On AGP 9+ the branch is skipped and Flutter provides Kotlin, unchanged.

This mirrors how plus_plugins fixed the same issue in their Kotlin-DSL packages: they moved KGP out of the plugins {} block into a conditional apply(plugin = ...) call that their regex likewise does not match. This is the Groovy equivalent.

Verification

Verified against Flutter's actual detection regexes (from flutter/flutter master): the previous line matched hasKgpPlugin (triggering the warning), the new file does not, while the required com.android.library detection still matches.

No Android toolchain is available in the dev environment, so this is a static verification against Flutter's detection logic rather than a full build run.

Notes

• CHANGELOG.md left untouched since it is generated from conventional commits via the release tooling.