UX polish tweaks to FallbackHome.

When showing during first boot (before provisioned), render as
completely black to avoid wallpaper jank before showing SUW.  Also
tweak theme used during normal (provisioned) launches to avoid
rendering the dim layer behind status bar.

Bug: 28530777
Change-Id: I4b9e6c54583f07c78d86fbe9bda7f24b6959f4ba