aac_afh: Don't create chunk tables any more.

aac_afh: Don't create chunk tables any more.

A previous commit activated dynamic chunks for the aac audio format
handler, so the virtual streaming system no longer consults the chunk
table stored in the audio file table of the osl database.

However, the code to open or add an audio file still assumes that
there is a chunk table and bails out if it can't find it. This patch
changes aft.c to do without a chunk table if the audio format handler
supports dynamic chunks. The afh_supports_dynamic_chunks() helper
needs to be made public because of this.

With chunk tables being optional, the ->get_file_info method of the
audio format handler can safely set ->chunk_table to NULL. It still
needs to compute the maximum chunk size though.