Implement PgUp/PgDn support. (cherry-pick DO NOT MERGE)
- Clean up our LayoutManager code. We actually only need a single
layout manager, since a GridLayoutManager is a LinearLayoutManager,
and list layout is simply grid layout with one column. Make it so.
- Use the layout manager to compute item offsets for one page's worth of
items. Use that to implement page up/page down.
BUG=
25195770
Change-Id: I23ddd5fd38532682e7f099b44e188d8c3fec02be
(cherry picked from commit
4d4bf9a21a16bae3497f94dbb558a2ede9d989ee)