Add bucket that unknown state playback falls into

MediaPlayerStressTest some media files are missing on the results as they
were reported neither completed nor erroneous.
This case will fall into a new category named unknown playback state so that
we could track them in media dashboard.

Bug: 21755950
Change-Id: Iefcc3e375c63d952a7d9edb418208209d87b9e08
2 files changed