Consider all Android versions, I prefer to
```
if (Build.VERSION.SDK_INT >= 16)
    mInstance = new XWalkMediaCodec(instance);
else (Build.VERSION.SDK_INT == 15)
     mInstance = new XWalkMediaCodecICS(instance);
else
    mInstance = new XWalkMediaCodecNull(instance);
```

Of coz, XWalkMediaCodecNull.java  need to be added. The content should be same with XWalkMediaCodecICSl.java  before we add 4.0 support in another PR.