BHV-18751: Prevent thumbs from showing when content is not scrollable.

### Issue
Due to some recent scroller changes, we were making a call to `showThumbs` for each `onScroll` event from ScrollMath. This caused the scroll thumb to be visible even when there was no content to scroll.

### Fix
We only make a call to `showThumbs` when we start scrolling, and make a call to `updateThumbs` for each ScrollMath `onScroll` event in order to hide as appropriate.

Enyo-DCO-1.1-Signed-off-by: Aaron Tam <aaron.tam@lge.com>