unify aac and mp3 audio format handlers

unify aac and mp3 audio format handlers

This makes the mp3 audio format handler use a chunk table just like
m4a and ogg vorbis already do. Moreover, the aac and mp3 audio format
handlers now have identical read_chunk and reposition_stream handlers.

Next step is to tweak the ogg vorbis audio format handler so that
it also fits into this scheme. In the end, this allows to move
read_chunk() and reposition_stream() from the audio format handlers
one layer up to the virtual streaming system and to kill the function
pointers in struct audio_format.

The advantage of this change that it avoids duplicated/similar code, and
that new audio format handlers can added easier.