mp4: Introduce mp4_is_audio_track().

Currently the aac audio format handler iterates over the tracks
in an mp4 file. For each track it tries to get the audio-specific
configuration by calling mp4_get_decoder_config() and calls into faad
to check whether it is a valid configuration for the aac decoder.

We can simplify all this because the mp4 code already knows the type
of each track, albeit it does not expose this information yet. So
provide the new mp4_is_audio_track() helper and let the aac audio
format handler pick the first track for which this helper returns true.

As an additional benefit, we can remove the now unused
mp4_get_decoder_config().