Output switcher subtitle modification

-Change from null to "Switch output"
-Update test case

Bug: 153834179
Test: make -j42 RunSettingsRoboTests
Change-Id: I184e78292e982f8a93b74612331a1d2cab28d07a

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

@@ -103,7 +103,7 @@ public class MediaOutputPanel implements PanelContent, LocalMediaManager.DeviceC
                 return metadata.getDescription().getSubtitle();
             }
         }
-        return null;
+        return mContext.getText(R.string.media_output_panel_title);
     }
 
     @Override