public sealed class LayerTrack : Track 

public void CreateLayerKeyframe(Layer layer, double transparency, TimeSpan atTime)
{
  var mapView = MapView.Active;
  if (mapView != null)
    return;

  var animation = mapView.Map.Animation;
  var layerTrack = animation.Tracks.OfType<LayerTrack>().First(); //There will always be only 1 LayerTrack in the animation.
  layerTrack.CreateKeyframe(layer, atTime, true, transparency, ArcGIS.Core.CIM.AnimationTransition.Linear);
}