Change IAudioTrackCallback buffer return values

The default implementation of IAudioTrackCallback for the data
callbacks should return 0 since we are not actually consuming data.
No clients using the associated transfer modes should be relying on
the base implementation, so this behavior should not be reached.

Comment cleanup.

Test: Compiles
Change-Id: Idff29d4d23de7c4ab8498491e8b41bcef8f9d3db