Debería el botón Back salirse de la aplicación?
Solo se puede utilizar en Android, Windows Phone o Windows Tablets.
By default this property is set to false, which means you're responsible for responding to Back button. You can do this by calling Input.GetKey and passing KeyCode.Escape.
If you set this property to true, clicking Back button:
* On Android will terminate the application.
* On Windows Phone or Table will suspend the application.