MediaSessionRecord: add synchronize control for modification of mControllerCallbackHolders

// ArrayIndexOutOfBoundsException thrown when unregisterCallback()
03-13 18:57:16.455  2112  6787 E JavaBinder: *** Uncaught remote exception! Exceptions are not yet supported across processes. Client PID 3726 UID 1000.
03-13 18:57:16.455  2112  6787 E JavaBinder: java.lang.ArrayIndexOutOfBoundsException: length=0; index=1
03-13 18:57:16.455  2112  6787 E JavaBinder:  at java.util.concurrent.CopyOnWriteArrayList.elementAt(CopyOnWriteArrayList.java:389)
03-13 18:57:16.455  2112  6787 E JavaBinder:  at java.util.concurrent.CopyOnWriteArrayList.remove(CopyOnWriteArrayList.java:482)
03-13 18:57:16.455  2112  6787 E JavaBinder:  at com.android.server.media.MediaSessionRecord$ControllerStub.unregisterCallback(MediaSessionRecord.java:1714)
03-13 18:57:16.455  2112  6787 E JavaBinder:  at android.media.session.ISessionController$Stub.onTransact(ISessionController.java:394)
03-13 18:57:16.455  2112  6787 E JavaBinder:  at android.os.Binder.execTransactInternal(Binder.java:1351)
03-13 18:57:16.455  2112  6787 E JavaBinder:  at android.os.Binder.execTransact(Binder.java:1282)

Change-Id: I02c3011ac8d27171016dd15ce7dbbadff4a2bb65
Signed-off-by: wumin3 <wumin3@xiaomi.com>