CastTile: Set media router group id to sync cast status

Using media router group id, CastTile can sync cast status with
Settings.
If Settings begins/stops cast (mirrorring), CastTile can know that and
Settings also knows when CastTile begins cast.

Please refer ag/7669852 to how router group id can be used to
synchronize cast status.

Bug: 112826114
Bug: 131385091
Bug: 130345243

Test: Manually tested after applying this and ag/7708778

Change-Id: Ie7c329220eb67725515517c950ff476cbee733c0

packages/SystemUI/src/com/android/systemui/statusbar/policy/CastControllerImpl.java

@@ -71,6 +71,7 @@ public class CastControllerImpl implements CastController {
     public CastControllerImpl(Context context) {
         mContext = context;
         mMediaRouter = (MediaRouter) context.getSystemService(Context.MEDIA_ROUTER_SERVICE);
+        mMediaRouter.setRouterGroupId(MediaRouter.MIRRORING_GROUP_ID);
         mProjectionManager = (MediaProjectionManager)
                 context.getSystemService(Context.MEDIA_PROJECTION_SERVICE);
         mProjection = mProjectionManager.getActiveProjectionInfo();