Below the sample code to add the mediaRouter listeners to the mediaRouter:

MediaRouter.Callback _cb = new MediaRouter.Callback() {

                    @Override
                    public void onRouteSelected(MediaRouter mediaRouter, int i, MediaRouter.RouteInfo routeInfo) {
                        Log.d("MediaRouteEvent:", "");
                        Log.d("onRouteSelected:", String.valueOf(_displayManager.getDisplays().length));
                    }

                    @Override
                    public void onRouteUnselected(MediaRouter mediaRouter, int i, MediaRouter.RouteInfo routeInfo) {
                        Log.d("MediaRouteEvent:", "");
                        Log.d("onRouteUnselected:", String.valueOf(_displayManager.getDisplays().length));
                        WritableMap wm = Arguments.createMap();
                        wm.putInt("displayCount", _displayManager.getDisplays().length);

                        sendEvent("routeEvent", wm);
                    }

                    @Override
                    public void onRouteAdded(MediaRouter mediaRouter, MediaRouter.RouteInfo routeInfo) {
                        Log.d("MediaRouteEvent:", "");
                        Log.d("onRouteAdded:", String.valueOf(_displayManager.getDisplays().length));
                    }

                    @Override
                    public void onRouteRemoved(MediaRouter mediaRouter, MediaRouter.RouteInfo routeInfo) {
                        Log.d("MediaRouteEvent:", "");
                        Log.d("onRouteRemoved:", String.valueOf(_displayManager.getDisplays().length));
                    }

                    @Override
                    public void onRouteChanged(MediaRouter mediaRouter, MediaRouter.RouteInfo routeInfo) {
                        Log.d("MediaRouteEvent:", "");
                        Log.d("onRouteChanged:", String.valueOf(_displayManager.getDisplays().length));
                        WritableMap wm = Arguments.createMap();
                        wm.putInt("displayCount", _displayManager.getDisplays().length);

                        sendEvent("routeEvent", wm);
                    }

                    @Override
                    public void onRouteGrouped(MediaRouter mediaRouter, MediaRouter.RouteInfo routeInfo, MediaRouter.RouteGroup routeGroup, int i) {
                        Log.d("MediaRouteEvent:", "");
                        Log.d("onRouteGrouped:", String.valueOf(_displayManager.getDisplays().length));
                    }

                    @Override
                    public void onRouteUngrouped(MediaRouter mediaRouter, MediaRouter.RouteInfo routeInfo, MediaRouter.RouteGroup routeGroup) {

                    }

                    @Override
                    public void onRouteVolumeChanged(MediaRouter mediaRouter, MediaRouter.RouteInfo routeInfo) {

                    }
                };
                _mediaRouter.addCallback(MediaRouter.ROUTE_TYPE_LIVE_VIDEO, _cb);

Cheers and Peace out!!!