audiopolicy: use deep buffer output by default for music streams

  Default disabled. This is enabled by
  adb shell setprop audio.deep_buffer.media 1

  Applications that use AudioTrack java interface for audio rendering
  defaults to primary output as flags indicated as FLAG_NONE. If the
  primary output is low latency output, it results higher power
  consumption than deep-buffer path if used.
  Make the deep buffer output as default option for music streams if
  flags are not specified.

  It may also be used for testing audio glitches from heavy CPU
  or DRM which may cause issues with the FastMixer.

Change-Id: I2afb205c15f73545168a8d1f68b29df01934da88