Navigation Page icon on Android

I can't appear to get the icon to display on a Navigation Page on a standard Xaml template in Visual Studio 2015 and 2017RC.

I add Icon="icon.png" to the Contentpage XAML and change App.Xaml.cs to:

MainPage = new NavigationPage( new Appicon.MainPage());

I get the navigation bar but it is empty when I deploy to an android device.

Am I missing something simple?