Fix invisible scroll thumb blocks clicks.

The current issue is that Views that have either the horizontal or
vertical scroll bars enabled will intercept mouse interactions that
entire the region where the scrolling thumb would be even if the View
cannot actually scroll because it's content isn't larger than it.

This is fixed by only intercepting mouse interactions in the scroll
thumb region if there is something to scroll.

Bug: 110375792
Test: None yet

Change-Id: Ib638b4ac88375f55bc80ba2a66d945a16ecd6d22