Bug 882099 - [Music] AAC Audio files with MP4 container format doesn't get scanned/displayed in Music player

r? @dominickuo

I've tried to refactor the MP4 metadata parsing a bit so that it's clearer. This seems to work on the samples I've tried, but I'm still testing against more unusual examples. If you find any issues, let me know. In particular, I have a few more supported ftyp brands than Gecko supports; should I remove those extra ones?