Implement PgUp/PgDn support.

- 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