Prepare for additional formats beyond 8-bit and 16-bit PCM

This CL does the same thing for AudioTrack,
that an earlier CL did for AudioRecord:
  > Change-Id: Ic3525f049e939bdf125d9f87ed39abd2690dcf9f

Pull out the common parts of that earlier CL and move them to new header
file android_media_AudioFormat.h.

Use the new function audioFormatToNative() to convert from Java
ENCODING_PCM_* to native AUDIO_FORMAT_*.

Use audio_bytes_per_sample() instead of hard-coding the size of various formats.

Use size_t for memory sizes.

Change-Id: I7992dd6a2c9500126a6d7ae5fc1ed9f3312962d6