Fix bug for direct track with PCM != 16-bit

The AUDIO_FORMAT_PCM_8_BIT format was being converted to
AUDIO_FORMAT_PCM_16_BIT on client side even for direct tracks.
That conversion was incorrect; it should only be done for mixed tracks.

Also remove checks for specific PCM formats in the generic part of
server side of createTrack.  Those format checks should only be done by
the thread.  This will allow direct tracks for PCM 8-bit, PCM 24-bit, etc.

Change-Id: If5b9fd79f8642ed93e2aeabcaf4809b2ed798978