Pull FlutterPlugin out of src/main/groovy/flutter.groovy and into its own class (src/main/Kotlin/FlutterPlugin.kt). Also, delete flutter.groovy, as this will need to be the last class (it depends on all the other classes in flutter.groovy, and we can only have Groovy code depend on Kotlin code (not vice versa) because of limitations of Gradle.

This will likely be significantly more difficult than other pieces of flutter.groovy. We may need to break out functionality into some new utils classes, if it is looking intractable to do it as one conversion.