handle error during flush in MediaPlayer.reset()

If there was an error during the flush phase of a reset, then the
reset would never complete. We now make sure the MediaCodec moves
to the right state in this case, and that NuPlayer cleans up and
resumes the rest of the reset after a failed flush.

Bug: 16955082
Change-Id: Ied61136871a9fcdffcc80647fa2bba64a926ac2a