Presently, our AAC decoder didn't skip any samples, unless the side data
contained skip information.

This uses the newly added ff_decoder_skip_samples function to skip samples
on two levels: base, and additional.
Base is the inherent decoder delay. Most containers already specify it,
hence it is overwritten in that case.
Additional is any extra samples that have to be skipped, in AAC's case,
it's the delay introduced by SBR.

Patch attached.