Fix full widgets picker fast scroll offset issue

Even though there is no change in fast scroller offset, the recycler view
scroll Y may still be updated. Let's also notify a scroll change if
the recycler view scroll has changed when updating the fast scroller thumb
offset.

Test: In the full widgets picker, scroll up and down with the fast
      scroller. Then, scroll up and down in the recycler view. The top
      section is scrolled together with the recycler view smoothly.
Bug: 183721076
Change-Id: I009fc7e27c8a20505a60502405ebbf695b5a37f2