chore(samples-android): Add optional SAGP build mode (#5538)

Adds an optional useSagp flag to Android sample app builds that, when true, applies the Sentry Android Gradle Plugin. (Defaults to false, which matches existing build behavior.)

```
./gradlew :sentry-samples:sentry-samples-android:installDebug -PuseSagp=true
```

See the Android sample app README for more details.