what is settings_aar.gradle in android/flutter for?

android flutter gradle

settings_aar.gradle is for AndroidX migration.

For older flutter version without AndroidX a long message should appear in CLI stdout when building the app after upgrade to newer flutter version

https://github.com/flutter/flutter/blob/54dba4cd59f4b330e66960f0d37cf57802f675a8/packages/flutter_tools/gradle/manual_migration_settings.gradle.md

https://github.com/flutter/flutter/blob/33aa45760430c6e6ea3552a263613f576c10c822/packages/flutter_tools/lib/src/android/gradle.dart#L154