Define new audio attribute flag: beacon

Flag AUDIO_FLAG_BEACON characterizes streams where the system
  ensures that the playback of the sound will be compatible
  with its use as a broadcast for surrounding people
  and/or devices. Ensures audibility with no or minimal
  post-processing applied.

Bug 15415971

Change-Id: I1df9dc50e3fcc973a0709a55df5b3bd95f81eb58