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>

media/java/android/media/MediaPlayer.java

@@ -2336,10 +2336,7 @@ public class MediaPlayer extends PlayerBase
     }
 
     private void scanInternalSubtitleTracks() {
-        if (mSubtitleController == null) {
-            Log.d(TAG, "setSubtitleAnchor in MediaPlayer");
-            setSubtitleAnchor();
-        }
+        setSubtitleAnchor();
 
         populateInbandTracks();