feat(android): Migrate Plugin to Built-in Kotlin

[EdwynZN]

Following #710 this fixes the use of AGP+9 and also when using android.builtInKotlin=false in gradle.properties (while android.builtInKotlin=false is discouraged to use in AGP9 it can still be false in this version until the migration is complete)

• Apply kotlin-android whenever built-in Kotlin is not active

[ened]

Thanks @EdwynZN — great catch, and your guard logic is the right one: AGP 9.0 enables built-in Kotlin by default, so treating the property as active unless explicitly false matches AGP's real semantics (a == 'true' check would wrongly apply KGP when the property is absent).

I pushed a small follow-up to your branch (comment only — it still said builtInKotlin=false means "no Kotlin plugin compiles at all", which this fix changes): 635ea4a.

Verified locally against the exact environment from #722 (AGP 9.0.1, KGP 2.3.20, android.newDsl=false, android.builtInKotlin=false): pre-fix evaluation fails with Could not find method kotlin() at line 60, post-fix BUILD SUCCESSFUL with the full plugin compiled into the AAR (74 classes incl. WorkmanagerPlugin/BackgroundWorker). builtInKotlin=true mode and the AGP 8.11 path (50 unit tests) also green.

Can you confirm this fixes the build for you? Also approved the CI runs for this PR.