Audioflinger: tracks monitor OP_PLAY_AUDIO

Mute/unmute tracks according to changes in OP_PLAY_AUDIO for
  the current usage.
In audio policy: always assign AUDIO_STREAM_ENFORCED_AUDIBLE
  to sonification tracks with AUDIBILITY_ENFORCED flag.
Do not mute tracks from root / audio server.
Do not mute UI sounds on AUDIO_STREAM_ENFORCED_AUDIBLE
  stream type.

Bug: 112339570
Test: enter DnD, play notifications, verify not heard
Change-Id: Ia5f1118481cf0573101acf2092fbd0ce2cf8c038