Add a category for unknown playback state

MediaPlayerStressTest some media files are missing on the results. Those
were reported neither completed nor erroneous.
This case needs to fall into a new category named unknown playback state
so that we could track on any files in this state.

Bug: 21755950
Change-Id: Ib45c41c74a0742e96a98fabf05ea900b769524c4
1 file changed
tree: c19fe9039e05ee251de69cd61831219e6b94db08
  1. src/