Refine divider behavior

- Switch earlier to the dismissing/fullscreen layout, so
we minimize holes.
- For the bottom/right one, switch also earlier to the larger size to
minimize holes. We can't do this for the upper one because it will be
confusing there if the layout bounds doesn't equal the bounds while
snapping.

Change-Id: I8f065514f7aaa15ae76af648794ab8b23302e534