Fix NullPointerException in MediaPlayer

Issue detail:
https://code.google.com/p/android/issues/detail?id=231417



setSubtitleAnchor() is a synchronized method and has checked
mSubtitleController. So checking for null pointer is not
needed in scanInternalSubtitleTracks() method.

Signed-off-by: qinzhichao <qinzhichao@xiaomi.com>