Implement issue #3201795: Improve transition when keyboard comes up

ViewRoot now does a fade animation between a snapshot of the previous
layout to the new one when its content rect changes.

Also tweaked some things in the window manager to fix problems in
deciding when to animate the movement of a window and when not to.

Change-Id: I9b4b3bd53c8258bd39a2f2fc315e77cfc56a409c