fix(wpf): going fullscreen then exiting doesn't go back to the previo…

…us window state

src/Uno.UI.Runtime.Skia.Wpf/UI/Controls/WpfWindowWrapper.cs

@@ -121,6 +121,7 @@ private void UpdateIsVisible()
 
 	protected override IDisposable ApplyFullScreenPresenter()
 	{
+		_isFullScreen = true;
 		_wpfWindow.WindowStyle = WindowStyle.None;
 		_wpfWindow.WindowState = WindowState.Maximized;