Never drag scroll views with no children. (DO NOT MERGE)

Fixes a regression from Froyo.  Previously, when a scroll view
had no children, its onTouchEvent would return because the scroll
view would only start dragging if the user touched one of its children.
In Gingerbread, the user can drag from anywhere within the scroll
view, not just by touching a child.  However, it makes no sense
to drag a scroll view that has no children so an empty scroll view
should just ignore touches like any other empty view group would.

This change fixes applications that for some reason or other happen
to have empty scroll views in strange places.

Bug: 3246230
Change-Id: I76c6136d3cd74968983033014d60deec1718ce30