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. AAC audio is also supported now. The build files on other OS are also updated.
PS: The issues in https://github.com/hrydgard/ppsspp/pull/5668 are still open questions, but never mind to do with this one.