I can see the problem here, the rendering mode for a GraphicsOverlay is set when you instantiate it. The default constructor will set it in DYNAMIC, but there is an overload which allows you to set this yourself.
The code you've written for changing the rendering mode is for feature layers.
Using the following code, I've managed to render your arrowhead:
PointCollection polylinePoints = new PointCollection(SpatialReferences.getWgs84());
polylinePoints.add(new Point(10, 10));
polylinePoints.add(new Point(20, 20));
Polyline polyline = new Polyline(polylinePoints);
SimpleLineSymbol lineSymbol = new SimpleLineSymbol(SimpleLineSymbol.Style.SOLID,
0xFF0063FF, 7, SimpleLineSymbol.MarkerStyle.ARROW, SimpleLineSymbol.MarkerPlacement.END);
Graphic polylineGraphic = new Graphic(polyline, lineSymbol);
GraphicsOverlay graphicsOverlay = new GraphicsOverlay(GraphicsOverlay.RenderingMode.STATIC);
sceneView.getGraphicsOverlays().add(graphicsOverlay);
graphicsOverlay.getGraphics().add(polylineGraphic);

I am however going to look at making the arrow head to render in the dynamic rendering mode for a future release.
Does this help?