Lock drm plugin API calls globally, not per MediaDrm instance

Change-Id: I062c662a440a1becccd248c3b8ddf711c51e53cc
related-to-bug: 18394494
related-to-bug: 19664283
diff --git a/media/libmediaplayerservice/Drm.h b/media/libmediaplayerservice/Drm.h
index c4013b8..056723c 100644
--- a/media/libmediaplayerservice/Drm.h
+++ b/media/libmediaplayerservice/Drm.h
@@ -143,7 +143,7 @@
     virtual void binderDied(const wp<IBinder> &the_late_who);
 
 private:
-    mutable Mutex mLock;
+    static Mutex mLock;
 
     status_t mInitCheck;