Clean up frame size in AudioTrack and AudioFlinger

TrackBase::mFrameSize, mChannelMask, and mChannelCount are now const.
Use TrackBase::mFrameSize instead of re-calculating frame size.
AudioFlinger only sees 16-bit PCM format, conversion from 8-bit is
  now entirely on the client side.  Previously a small part of the
  responsibility was on server side also.
size_t is unsigned, so use %u in logs.
Fix theoretical bug where TrackBase constructor was over-allocating space
  for non-linear AudioTrack or 8-bit PCM AudioRecord (probably benign).

Change-Id: I7cbbba0bf4dba29ea751d8af341ab8e5cbbdc206