Support speaker routing for FLAG_BEACON

Add support for routing audio stream to the speaker when audio
  attributes carry the FLAG_BEACON value:
- associate streams with FLAG_BEACON to the up-to-now unused
  STREAM_TTS ("Transmitted Through Speaker").
  The remapping happens in AudioTrack.
- AudioPolicyManager: define new strategy for handling the Beacon
  behavior, that only goes to speaker. Define new volume curves
  for STREAM_TTS that are not silence only for the speaker device
  category. Keep ref count of other streams and never play Beacon
  streams when anything else is playing: either don't start playback
  if something else is playing, or unmute STREAM_TTS when it's
  allowed to play and mute when it's done.
- Take into account muting latency when starting an output.

Bug 15415971

Change-Id: I26539c7ec1a486accd85bbeb1623e9c7d3a1192f