Update project to AndroidX

[fezu54]

Is it possible to update the Android project to use the AndroidX libraries instead of the old support libraries like described here https://flutter.dev/docs/development/packages-and-plugins/androidx-compatibility#how-to-migrate-a-flutter-app-to-androidx ?

This would enable projects which are dependent on this one to migrate to AndroidX too (like ours 😊 )

[jamieastley]

@fezu54 I've just updated the package to AndroidX and made a pull request, give it a test and make sure it works for you -> #28