fix for android5 with nexus5 background music play

the original can't play background music.
1. you need to stop preload bgm
2. whether or nor the same bgm file , always create new MediaPlayer

my java is pool , but it base on and can work well.
https://github.com/Dhilan007/cocos2d-x/blob/76c9accec9b8c3884f9ddec83be44df5d828605b/cocos/platform/android/java/src/org/cocos2dx/lib/Cocos2dxMusic.java