public void SetVisibility( 
   bool isVisible
)

if (!layer.IsVisible)
  layer.SetVisibility(true);

if (layer is FeatureLayer featureLayer)
{
  if (!featureLayer.IsEditable)
    featureLayer.SetEditable(true);

  if (!featureLayer.IsSnappable)
    featureLayer.SetSnappable(true);
}

//Must be on the QueuedTask
var url = "https://geoeventsample1.esri.com:6443/arcgis/rest/services/AirportTraffics/StreamServer";
var createParam = new FeatureLayerCreationParams(new Uri(url))
{
  IsVisible = false //turned off by default
};
var streamLayer = LayerFactory.Instance.CreateLayer<StreamLayer>(createParam, map);

//or use "original" create layer (will be visible by default)
Uri uri = new Uri(url);
streamLayer = LayerFactory.Instance.CreateLayer(uri, map) as StreamLayer;
streamLayer.SetVisibility(false);//turn off