Audio focus: definitive loss of focus removes client from stack

Use annotation for locking instead of javadoc comments.
Better javadoc for focus loss propagation.
When an audio focus user loses focus with AUDIOFOCUS_LOSS,
  remove it from the focus stack to prevent improper
  app behaviors (e.g. starting to play when regaining focus
  after a definitive loss).

Test: 1/ launch media playback app that uses audio focus
         and play music
      2/ launch other media app that requests AUDIOFOCUS_GAIN
         and play media
      3/ "adb shell dumpsys audio" verify first app is not
         in focus stack anymore
Change-Id: I2d4fb8ab8e554a38a650ddce440cc2f315083d08