Drawing shapes in Android MapView independent of the zoom level

I need to draw shapes just like those you would create with custom maps on Google Maps, using Android's MapView.

In other words, if I draw a shape on the map, when I zoom out it should shrink, covering the same area of the map regardless of the zoom level.

Example following the Android Reference:

  @Override
  public boolean draw(Canvas canvas, MapView mapView, boolean shadow, long when)
  {
      super.draw(canvas, mapView, shadow);

      //---translate the GeoPoint to screen pixels---
      Point screenPts = new Point();
      mapView.getProjection().toPixels(p, screenPts);

      Paint boxPaint = new Paint();
      boxPaint.setColor(android.graphics.Color.WHITE);
      boxPaint.setStyle(Paint.Style.FILL);
      boxPaint.setAlpha(140);
      canvas.drawCircle(screenPts.x, screenPts.y, 20, boxPaint);

      return true;
  }

This shows a white circle on the map, but if you zoom out, the circle is the same size. Perhaps using canvas is not right approach?

I need something like how Google Maps highlights neighborhoods or cities:

Any ideas? Thanks in advance!