App.cs

public App()
        {
            this.InitializeComponent();
            this.Suspending += this.OnSuspending;
            HardwareButtons.BackPressed += HardwareButtons_BackPressed;
        }

        private void HardwareButtons_BackPressed(object sender, BackPressedEventArgs e)
        {
            var rootFrame = Window.Current.Content as Frame;

            if (rootFrame != null && rootFrame.CanGoBack)
            {
                e.Handled = true;
                rootFrame.GoBack();
            }
        }