Serialize calls to getFrameAtTime in the mediaserver

to avoid running out of memory.

Bug: 21277449
Change-Id: I7210806a24958f38d86aadab0310fc263e4c0edf
diff --git a/media/libmediaplayerservice/MetadataRetrieverClient.h b/media/libmediaplayerservice/MetadataRetrieverClient.h
index e71a29e..fe7547c 100644
--- a/media/libmediaplayerservice/MetadataRetrieverClient.h
+++ b/media/libmediaplayerservice/MetadataRetrieverClient.h
@@ -63,6 +63,7 @@
     virtual ~MetadataRetrieverClient();
 
     mutable Mutex                          mLock;
+    static  Mutex                          sLock;
     sp<MediaMetadataRetrieverBase>         mRetriever;
     pid_t                                  mPid;