Race-condition in SoundPool during release

There is race between SoundPoolThread and SoundPool / AudioManager
threads during releasing SoundPool.
AudioManager deletes a global reference before setting SoundPool
callback to NULL. If, at that time, a call to the SoundPool::notify
fuction happens then mCallback is valid but mUserData is not.

The following log will show up to indicate the problem:
  JNI ERROR (app bug): accessed deleted global reference 0xXXXXXXXX

This fix is to clear the SoundPool's callback before releasing global
reference.

Change-Id: I5e6d647edc0444340db879428048e2c0a068a8b4