Add dump to MediaSessionService

Adds a dump implementation for debugging MediaSessionService. Also
fixes some synchronize calls that weren't using the same lock object.

Change-Id: I14343f853398749c8ce7ebf91f72729abc9132d9

services/java/com/android/server/SystemServer.java

@@ -906,7 +906,6 @@ public final class SystemServer {
             }
 
             try {
-                Slog.i(TAG, "MediaSessionService");
                 mSystemServiceManager.startService(MediaSessionService.class);
             } catch (Throwable e) {
                 reportWtf("starting MediaSessionService", e);