Fix fast scroller issues / remove fast thumb access / fix wrong popup location

(1) Remove jumping to arbitrary fast scroller position
because back gesture can trigger arbitrary fast scrolling
(2) Pop up would show up in random location when user combines
touching the track and scrolling. This was because thumb
location was not updating when user was scrolling on the main
container.

Bug: 193177670
Bug: 191562400

Test: manual
Change-Id: I129aaa37ca911666453a8c98e24eaac33827238f