aacdec: Rework channel mapping compatibility hacks.

For a PCE based configuration map the channels solely based on tags.
For an indexed configuration map the channels solely based on position.

This works with all known exotic samples including al17, elem_id0, bad_concat,
and lfe_is_sce.

Originally committed as revision 25098 to svn://svn.ffmpeg.org/ffmpeg/trunk
