Use one SimpleAudioDec to decode all supported audio AT3, AT3+, MP3, AAC...

I try to combine the two similar SimpleMp3Dec and SimpleAT3Dec into one, named SimpleAudioDec. This aims to support all audio decode AT3, AT3+, MP3, AAC ... in one structure.
So, we can completely remove SimpleMp3Dec and SimpleAT3Dec from project now.

Test on windows is just ok. Should update the other targeting project too.
PS: The issues in https://github.com/hrydgard/ppsspp/pull/5668 are still open questions, but never mind to do with this one.