NuPlayer: fix av discontinuity race

Assumption:
  NuPlayer must see both (a/v) discontinuities before resuming.

Example issue:
  Suppose we're changing the video track, so the video source queues a
  DISCONTINUITY_FORMATCHANGE, and the audio source queues a DISCONTINUITY_NONE.
  Consider this sequence of events without this change:
    1. audio discontinuity seen by player
    2. mFlushingAudio = FLUSHED
    3. video discontinuity seen by player
    4. mFlushingAudio != NONE so no performScanSources is queued
    5. video decoding shuts down without being restarted

Bug: 15153976
Change-Id: I659550d9c0f883828103142abead857f576f25b1