Fixed some issues with fullscreen dimming

- Always dim home stack and task fullscreen. The home stack could be
in split-screen mode, but we still want to dim dialogs that are
associated with the stack or task in fullscreen. The dialogs are not
really activities, but they are associated with the home stack for
things like dimming. Don't ask me why...
- Update the fullscreen dim layer bounds anytime the dim layer is
adjusted so we always have up-to-date bounds after rotation.

Bug: 28575624
Change-Id: I805c771153a2d25fb199bd9987bbf78a5967f6b9