title: Replace Xamarin forms page without pushasync
body: <p>I've searched through numerous online posts and can't find a suitable answer for this scenario. It's a pretty common requirement. I have a Xamarin.Forms app with a login page and listing page.  When the app is started, the login page is displayed.  When they login successfully, I want to display the listing page and prevent them from backing up by clicking on the toolbar back button or the hardware back button.</p>