8460 Android Embedding PR25: Prevent black rectangle when launching FlutterActivity (cla: yes)

8461 Log the correct function on error in the embedder. (cla: yes)

8462 Document the leak_vm flag. (cla: yes)

8465 Android Embedding PR26: Offer an async version of FlutterMain’s ensure initialization complete. (cla: yes)

8467 Initialize OpacityLayer’s matrix to identity (cla: yes)

8472 [Docs] Correcting link to contributing guide. (affects: docs, cla: yes, easy fix)

8473 Roll dart back to 907c514c8937cf76e (cla: yes)

8477 Add trace events for creating minikin fonts (cla: yes)