If you’re using Android plugin for Gradle 3.0.0 or higher, your project
automatically uses the default version of the build tools that the
plugin specifies. Alternatively, you can specify a version of the build tools.

Under the android block

Specify the API level Gradle should use to compile your app.
Defaults to flutter.compileSdkVersion.

For more information, see the module-level build section in the Gradle build file.

Building the app for release

You have two possible release formats when publishing to
the Play Store.

App bundle (preferred)

APK

Note:
  The Google Play Store prefers the app bundle format.
  For more information, see About Android App Bundles.