Do not source control the .android/ directory
  since it’s autogenerated. Before building the
  module on a new machine, run flutter pub get
  in the my_flutter directory first to regenerate
  the .android/ directory before building the
  Android project using the Flutter module.

Note:
  To avoid Dex merging issues, flutter.androidPackage should not be identical to your host app’s package name

Java 11 requirement

The Flutter Android engine uses Java 11 features.

Before attempting to connect your Flutter module project
to your host Android app, ensure that your host Android
app declares the following source compatibility within your
app’s build.gradle file, under the android { }
block, such as:

android

//...

compileOptions

sourceCompatibility

11

targetCompatibility

11

Add the Flutter module as a dependency