It is decoder-only; furthermore, there is already
an AACContext in use by libfdk-aacenc.
Also make aacdec.h provide the typedef for AACContext;
up until now, this has been done by sbr.h.
Reviewed-by: Lynne <dev@lynne.ee>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>