Only works for Android 4.2 or higher. The android renderer for the ContentViewRoundedCorners control.

ContentViewRoundedCornersRenderer.cs

class ContentViewRoundedCornersRenderer : VisualElementRenderer<ContentView>
{
    private float _cornerRadius;
    private RectF _bounds;
    private Path _path;

    protected override void OnElementChanged(ElementChangedEventArgs<ContentView> e)
    {
        base.OnElementChanged(e);

        if (e.OldElement != null)
        {
            return;
        }

        var element = (ContentViewRoundedCorners) Element;

        _cornerRadius = TypedValue.ApplyDimension(ComplexUnitType.Dip, element.CornerRadius,
            Context.Resources.DisplayMetrics);
    }

    protected override void OnSizeChanged(int w, int h, int oldw, int oldh)
    {
        base.OnSizeChanged(w, h, oldw, oldh);
        if (w != oldw && h != oldh)
        {
            _bounds = new RectF(0, 0, w, h);
        }

        _path = new Path();
        _path.Reset();
        _path.AddRoundRect(_bounds, _cornerRadius, _cornerRadius, Path.Direction.Cw);
        _path.Close();
    }

    public override void Draw(Canvas canvas)
    {
        canvas.Save();
        canvas.ClipPath(_path);
        base.Draw(canvas);
        canvas.Restore();
    }
}