fix(battery_plus): apply KGP unless built-in Kotlin is enabled

[4akloon]

Description

battery_plus's Android build script decided whether to apply the Kotlin Gradle Plugin (KGP) from the AGP major version alone:

if (agpMajor < 9) {
    apply(plugin = "org.jetbrains.kotlin.android")
}

That assumes AGP 9 always means built-in Kotlin is active. AGP 9 enables built-in Kotlin only when android.builtInKotlin is not explicitly false — and android.builtInKotlin=false is exactly what flutter create writes into every new app's android/gradle.properties, and what this repository's example apps set.

So on AGP 9 + android.builtInKotlin=false the plugin skipped KGP and AGP provided no Kotlin either. Nothing compiled the plugin's Kotlin sources, and configuration failed a few lines below at the unconditional extension lookup:

Extension of type 'KotlinAndroidProjectExtension' does not exist.

This is not visible today only because Flutter's FlutterPluginUtils.kgpRegexKotlin matches plugins { } blocks but not the imperative apply(plugin = "…") form used here, so Flutter does not see the declaration and applies KGP itself — accidentally papering over the broken guard. Flutter already warns that this fallback is going away.

This PR guards on the same condition AGP itself uses:

// AGP 9 provides Kotlin support natively unless the consuming app opts out with
// android.builtInKotlin=false, which is what `flutter create` writes by default.
val builtInKotlinEnabled =
    agpMajor >= 9 &&
        (providers.gradleProperty("android.builtInKotlin").orNull?.toBoolean() ?: true)

if (!builtInKotlinEnabled) {
    apply(plugin = "org.jetbrains.kotlin.android")
}

The KotlinAndroidProjectExtension block below is unchanged — AGP 9's built-in Kotlin registers that extension, so it resolves under both branches. On AGP 8 behaviour is provably identical to today: agpMajor >= 9 is false, so KGP is applied exactly as before.

Full mechanism, AGP bytecode evidence and reproduction steps are in #3944.

Verification

Scenario	Result
AGP 9.1.0 / Kotlin 2.4.0 / Gradle 9.3.1, builtInKotlin=false, with Flutter's fallback regex patched out, before fix	Fails with KotlinAndroidProjectExtension does not exist
Same, after fix	flutter build apk --debug succeeds
AGP 9, builtInKotlin=true	Succeeds; KGP correctly not applied
Stock AGP 8.12.1 (current pin)	Succeeds

melos run analyze and melos run test:unit_all both pass.

Related Issues

• Related Android plugins apply KGP based on AGP major version only, breaking on AGP 9 with android.builtInKotlin=false #3944

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the plugin version in pubspec.yaml files.
• All existing and new tests are passing.
• The analyzer (flutter analyze) does not report any problems on my PR.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.

[ajosephau]

Confirming this downstream. We hit exactly this on a Flutter app with AGP 9: flutter create writes android.builtInKotlin=false, so the agpMajor < 9 guard skips applying KGP while AGP doesn't supply built-in Kotlin either - leaving neither, and the build dies with UnknownDomainObjectException: Extension of type 'KotlinAndroidProjectExtension' does not exist. Checking the property rather than inferring from the AGP version is the right fix.

We're currently pinning a fork carrying just this commit and #3950 so we can move to AGP 9.3.2. With both applied, verified green: assembleDebug, assembleRelease (R8 full mode), and unit tests across the app and a Wear module. Reverting either reproduces the error.

FWIW the failing checks (as of the time I wrote this comment) here look unrelated to the change: they're iOS/macOS/Windows jobs failing on Unable to load contents of file list: '/Target Support Files/Pods-Runner/...xcfilelist' (a CocoaPods PODS_ROOT path issue). While every Android check passes (including integration tests on API 24/29/32/36) other open PRs show the same 5 failures, so it looks repo-wide rather than PR-specific.