flutter.jar/assets/flutter_shared will not merge into aar's assets path.

Consult [Add Flutter to existing apps](https://github.com/flutter/flutter/wiki/Add-Flutter-to-existing-apps#experiment-turn-the-flutter-project-into-a-module)

I use ./gradlew flutter:assembleDebug to assemble `flutter-debug.aar`  and add it into my project.

`flutter.jar/assets/flutter_shared` was not merged into `flutter-debug.aar` which cause my app crash .

