mp4: Hide tracks array.

mp4: Hide tracks array.

All functions of mp4.c operate on the first audio track. This
patch makes this fact implicit which allows us to remove the public
mp4_get_total_tracks() and mp4_is_audio_track(). Moreover, the track
parameter can be removed from all public functions.

If no audio track was found in the mp4 file, we now return an error
from two public open functions of mp4.c. Otherwise, we maintain a
pointer to the first audio track within the mp4 structure and use
that to identify the track rather than letting the API users pass
the track number.