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.


Start by locating the paper-toast component's responsive positioning and animation implementation, then reproduce the behavior at the xs and sm breakpoints. Done means a toast that becomes bottom-aligned on small screens uses an animation direction consistent with that final position, regardless of its requested desktop position.

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