Move fast scrolling logic to BaseRecyclerView

- This change has no effect on actual functionality but to make it easier
for widget tray to inherit the goodness of the 1) fast scroller
functionality 2) unify the scroll look and feel with the all apps view

b/21375339

Change-Id: Ib859b1c3352c0b69f16549ded8f20eb82cf58ba8