paper-toast: Slides in from wrong direction on small screens

On small screens (`xs`, and I think also `sm`), paper-toasts automatically go full-width and move to the bottom regardless of where you requested it to appear. This matches angular-material and is perfectly acceptable - except that although the position and size change, the animation doesn't - if the toast would have appeared at the top, it still slides "down" when appearing and "up" when disappearing.