Enable Multithreaded Rendering for Android

[threeseed]

This issue has now been closed:
flutter/flutter#14169

So it should be now possible to support multithreaded rendering.

[simolus3]

Thanks for the info. Unfortunately, enabling multithreaded rendering still crashes the example app, seems like we're running into another issue here :(
I'll try building a smaller native plugin reproducing that behavior and will report it to Flutter.

Edit: It seems like this is not an issue with Flutter, but multithreading rather crashes the native skia library of android when the animations are drawn. I'm still not sure what's causing this though.