Output switcher header is not updated with the latest media content

-Register media controller callback
-Update panel header when medadata changes
-Add test cases

Bug: 151396986
Test: make -j50 RunSettingsRoboTests
Change-Id: Id0b8f997899a96c0c94ca6bf099e624a9e87795b

src/com/android/settings/panel/PanelContentCallback.java

@@ -28,8 +28,8 @@ public interface PanelContentCallback {
     void onCustomizedButtonStateChanged();
 
     /**
-     * It will be called when group content is changed. For example, to add/remove a device into
+     * It will be called when header content is changed. For example, to add/remove a device into
      * a group
      */
-    void onGroupChanged();
+    void onHeaderChanged();
 }