The Cast volume UI doesn't show up

-Add MediaRouter2Manager::startScan() in startScan()

Bug: 176774510
Test: make -j50 RunSettingsLibRoboTests ROBOTEST_FILTER=InfoMediaManagerTest
Change-Id: I16f0528bf6daa2803e6ac8ec86f302cd24dd471d

packages/SettingsLib/src/com/android/settingslib/media/InfoMediaManager.java

@@ -84,12 +84,14 @@ public class InfoMediaManager extends MediaManager {
     public void startScan() {
         mMediaDevices.clear();
         mRouterManager.registerCallback(mExecutor, mMediaRouterCallback);
+        mRouterManager.startScan();
         refreshDevices();
     }
 
     @Override
     public void stopScan() {
         mRouterManager.unregisterCallback(mMediaRouterCallback);
+        mRouterManager.stopScan();
     }
 
     /**