[kevmo314]

Fixes the Android build warnings that come from our own build files and the on_device_tts plugin.

Merge order: merge this after the AudioService foreground-service PR. AudioService.kt imports androidx.multidex.BuildConfig, which only resolves because of the multidex dependency removed here. Without that PR, :app:compileDebugKotlin fails with Unresolved reference 'BuildConfig'. Note for that PR: the app's own com.rtirl.chat.BuildConfig isn't generated either, because AGP turns buildFeatures.buildConfig off by default. So the fix there needs a string literal or buildFeatures { buildConfig = true }.

android/gradle.properties

• Removed android.enableJetifier=true. :app:dependencies shows no com.android.support or android.arch artifacts in the debug or release runtime classpaths.
• Added org.gradle.caching=true.
• Kept android.builtInKotlin=false and android.newDsl=false because the build fails without them:
  • Without newDsl=false, the Flutter Gradle plugin (3.47.7) fails to apply because it casts to the legacy AbstractAppExtension.
  • Without builtInKotlin=false, flutter_web_auth_2 (and the firebase_* / cloud_functions plugins) still apply kotlin-android, and AGP 9 refuses that.
  • While builtInKotlin=false is set, the Flutter Gradle plugin applies kotlin-android to :app itself. So the "Deprecated 'org.jetbrains.kotlin.android' plugin usage" warning for :app and the kotlin-android entry in settings.gradle both have to stay until those upstream plugins migrate.

android/app/build.gradle

• Removed the multidex dependency and multiDexEnabled true. The Flutter default minSdk is 24.
• Removed the duplicate namespace.
• Converted the propName value assignments to propName = value (Gradle 10 removes the old form). compileSdkVersion, minSdkVersion and targetSdkVersion become compileSdk, minSdk and targetSdk.

packages/on_device_tts

• build.gradle.kts: android {} becomes configure<com.android.build.api.dsl.LibraryExtension> {}, and srcDirs(...) becomes directories.add(...). These were the last 3 script-compile warnings.
• SystemTtsPlugin.kt: removed 2 unnecessary !!. The plugin already uses AudioFocusRequest on API 26+. Because minSdk is 24, the pre-26 fallback stays, with a scoped @Suppress("DEPRECATION").

Warning counts, from ./gradlew assembleDebug --warning-mode all --rerun-tasks before and after. AudioService's import was patched locally for the "after" run and is not part of this PR.

	before	after
AGP WARNING: lines	13	11 (multidex and jetifier gone)
Gradle deprecation lines	52	46
"propName value" deprecations	13	9 (all from pub-cache plugins)
Kotlin compiler problems	66	62
on_device_tts build.gradle.kts script warnings	3	0

Nothing that remains comes from our own files. It's all from the Flutter Gradle plugin and third-party plugins in the pub cache:

• legacy variant API calls
• kotlin-android deprecations
• Project.getProperties
• project-object dependency notation
• deprecations in plugin Kotlin code
• the warning in AudioService.kt, which is another PR's file

Checked (with the AudioService import patched locally):

• flutter build apk --debug
• ./gradlew assembleDebug
• ./gradlew :on_device_tts:testDebugUnitTest
• flutter analyze (no issues)
• flutter test (467 passed)
• flutter test in packages/on_device_tts (95 passed)