Mark off-screen pages as unimportant for accessibility

ag/2458689 reduced page spacing, making it possible for
adjacent pages to overlap the current viewport. Though
you can't see this overlap due to paddings, this caused
items on the edges of adjacent pages to be focusable in
accessibility.

Bug: 63335071
Change-Id: I1296e7a711e2f0af8fc8bedda70a94a7693c7055