Delegate Taskbar touches to nearest view to ensure 48x48dp touch size

In TaskbarView#onTouchEvent(), which is only reached if a Taskbar
icon didn't already consuem the event, check each child to see if
the event occurs within a 48x48dp bounding box, and delegate the
event and subsequent events to it until UP or CANCEL.

Bug: 171917176
Change-Id: I7afafe0835828ab9213ec6abfe4e88ad7b9af3c4