qualities: rank aac lower than ac3

When the audio track is encoded with AAC, it is not uncommon for it to
be stereo only. On the other hand, the DD5.1 tag usually translates to
an AC3 audio track.

Therefore, rank an AAC encoded audio track below a DD5.1 audio track.
Merge pull request #390 from vincentbernat/fix/qualities-aac-priority

qualities: rank aac lower than ac3