Fix FastScroller regression for non-UI threads

Previously it was safe to call setFastScrollEnabled() and
setFastScrollAlwaysVisible() off the UI thread. After switching
FastScroller to use an Overlay, these methods stopped working.

This change ensures that all direct interaction with FastScroller
happens on the thread that created the host AbsListView.

BUG: 10210504
Change-Id: Ib6d9bd9212965420c0de39546652e3bc2d32ff8b