Make scroll bar mouse draggable.

This is for Views that have special mouse dragging handling. e.g.
TextView invokes text selection on mouse dragging, so it cannot be
scrolled by mouse dragging.
This provides such Views or ViewGroups a last resort to scroll as we
don't assume that all devices have touch panel, touch pad, or mouse
wheel.

Bug: 20016455

Change-Id: I68a13258a50b5e4ea681b2576da6000a0bb3fa65