Fix issue #6037252: Screen shifts after all apps are upgraded

Two things: (1) make sure the boot message is always positioned within
the entire unrestricted display, and (2) allow the dim background to go
on top of the nav bar when being used for the boot message (this latter
is really a hack that should be more generally fixed in the future).

Change-Id: I7261b044eb802a39cadff931b50a679ff18781d6