pbutils: add codec-specific utility functions for AAC

pbutils: add codec-specific utility functions for AAC

This allows us to add generic codec-specific functionality, like
extracting profile/level data from headers, without having to duplicate
code across demuxers and typefindfunctions.

As a starting point, this moves over AAC level extraction code from
typefindfunctions, so it can be reused in qtdemux, etc.

https://bugzilla.gnome.org/show_bug.cgi?id=617314

API: gst_codec_utils_aac_get_sample_rate_from_index()
API: gst_codec_utils_aac_get_level()