The current area covered by your mapview is the ViewPoint.
You can read the Viewpoint into an Envelope geometry. From that Envelope you can call getCenter() which is the centre point of your screen
Envelope extent = (Envelope) mapView.getCurrentViewpoint(Viewpoint.Type.BOUNDING_GEOMETRY).getTargetGeometry();
System.out.println("extent " + extent.toString());
System.out.println("centre " + extent.getCenter().toString());
Is this what you are after?