Ringtone: clear the MediaPlayer listener upon completion

This breaks a local refcount cycle and allows the Java gc to finalize
the Ringtone and MediaPlayer objects quickly.

Bug: 31811091
Change-Id: I0e5a4a78fdc02cc0c8b7811a1f1a45db3c90a0ff

media/java/android/media/Ringtone.java

@@ -469,6 +469,7 @@ public class Ringtone {
             synchronized (sActiveRingtones) {
                 sActiveRingtones.remove(Ringtone.this);
             }
+            mp.setOnCompletionListener(null); // Help the Java GC: break the refcount cycle.
         }
     }
 }