Support hardware media buttons on Gearhead sample.

Bug: 17813653
Change-Id: I094f67931ad02d7cf29ae90a0917c435f302cada
diff --git a/MusicDemo/src/main/java/com/example/android/musicservicedemo/MusicService.java b/MusicDemo/src/main/java/com/example/android/musicservicedemo/MusicService.java
index 642d3e2..f7e9dfc 100644
--- a/MusicDemo/src/main/java/com/example/android/musicservicedemo/MusicService.java
+++ b/MusicDemo/src/main/java/com/example/android/musicservicedemo/MusicService.java
@@ -181,6 +181,8 @@
         mSession = new MediaSession(this, "MusicService");
         setSessionToken(mSession.getSessionToken());
         mSession.setCallback(new MediaSessionCallback());
+        mSession.setFlags(MediaSession.FLAG_HANDLES_MEDIA_BUTTONS |
+                MediaSession.FLAG_HANDLES_TRANSPORT_CONTROLS);
         updatePlaybackState(null);
 
         mMediaNotification = new MediaNotification(this);