Fix #3398 ,when click backkey,it crashes 

on android,if u click the back key,it will crash.

11-06 23:09:45.158: E/libEGL(9627): call to OpenGL ES API with no current context (logged once per thread)

the problem is that u can't make native call for onBackPressed,it should be called in navtive GL thread,not in Android thread.
I fix this bug.
I have tested this!