ao_coreaudio: fix channel mapping

There where 3 major errors in the previous code:

1) The kAudioDevicePropertyPreferredChannelLayout selector returns a single
   layout not an array.
2) The check for AudioChannelLayout allocation size was wrong (didn't account
   for variable sized struct).
3) Didn't query the kAudioDevicePropertyPreferredChannelsForStereo selector
   since I didn't know about it's existence.

All of these are fixed.

Might help with #1367