Remove default channel mask for AudioTrack

It doesn't make sense to have a default channel mask, since the caller
needs to know what format it will use when supplying data, and there
is currently no API to return the channel mask of an AudioTrack.
audio_is_output_channel() does not allow 0, so it will catch any
stragglers (I'm not aware of any).

Also move channel mask validation earlier.

Change-Id: Ia018ded8711455581a2a935f37432b049422d492