Note:
  Astute readers might notice that the Flutter module
  directory also contains an .android and an
  .ios directory. Those directories are Flutter-tool-generated
  and are only meant to bootstrap Flutter into generic
  Android or iOS libraries. They should not be edited or checked-in.
  This allows Flutter to improve the integration point should
  there be bugs or updates needed with new versions of Gradle,
  Android, Android Gradle Plugin, etc.

For advanced users, if more modularity is needed and you must
  not leak knowledge of your Flutter module’s dependencies into
  your outer host app, you can rewrap and repackage your Flutter
  module’s Gradle library inside another native Android Gradle
  library that depends on the Flutter module’s Gradle library.
  You can make your Android specific changes such as editing the
  AndroidManifest.xml, Gradle files or adding more Java files
  in that wrapper library.

C. Merging libraries