Fix ViewRootImpl handling of content changes, fix ViewPager parenting

Previously content changes were only handled if they came directly from
the focused host, which meant that changes that occurred higher in the
tree were ignored. As a result, scrolling in ViewPager that contained
virtual nodes would fail to update the focus position.

Change-Id: I028bd8c670f1210339da331626e1986c5b5d4b87