Make sure all overlay panels are visible as user free scrolls.

Previously there was a workaround to ensure that adjacent panels were visible
while in the overview or spring-loaded states, but it incorrectly kept only
those original pages visible even while the user scrolled to other pages. So now
we only use the workaround when first entering the overview or spring-loaded
states, and then fall back to the default getVisiblePages() implementation in
PageView when in free scoll mode.

Bug: 23766408
Change-Id: I692ec00b9cd6d7889c374aee41b85abd0a5d8d3c
3 files changed