Universal audio branch

Based on my implementation in sceAac https://github.com/hrydgard/ppsspp/pull/5836
I've created a class AuCtx included in my SimpleAudioDec.cpp/.h which aims at providing a standard and easy implementation to support all codecs in ffmpeg for ppsspp.

Here, I also completely re-code sceMp3 file very fast in using this class and give you as an example to show how to use this class, and also very glad to see that it has almost solved all mp3 issues I've observed in the current master.
Tests on different freq and channels mp3 audios are all right:
Miku custom BGM (48kHz, stereo), Hanayaka Nari Wa ga Ichizoku(32kHz, mono, a little fast but better than before now), downstreet panic (44.1kHz, stereo), and learn jp09(44.1kHz, stero).

Especially, I am very glad to see that Miku's Custom BGMs have no repetition issues in the first tone any more, and no longer stopped in the first second neither :)

We could be coming into a new age of fast implementing new audio formats from now on I hope :D
PS: save/loadstate is also all right.