AudioMixer uses downmix effect for multichannel content

In the AudioMixer structure associated with each track, add an object
  that acts as the buffer provider when the track has more than two
  channels of input in the mixer. This object, DownmixerBufferProvider,
  gets audio from the actual buffer provider of the track, and applies
  a downmix effect on it.
The downmix effect is created and configured when the track gets
  created in AudioFlinger, which causes AudioMixer::getTrackName()
  to be called with the new track's channel mask. It is released
  when the track is disabled in the mixer.

Change-Id: I05281ed5f61bef663a8af7ca7d5ceac3517c82db