Fix assorted FastScroller regressions

Fixes the following bugs:
- Don't show preview if text is empty, e.g. whitespace
- Avoid transitioning decor from DRAGGING to DRAGGING
- Restore original font size
- Restore thumb position adjustment for last row

BUG: 11207575
Change-Id: I72fc031d1fd22a41d81e7f52ca0856cf82d1be27