Fix issue where Workspace scrolling doesn't work

=> The WorkspaceTouchListener was relying on receiving TOUCH_DOWN in order to
   clear its state. This is not guaranteed; ie. when touches begin above
   children of the workspace (icons or widgets)

=> Any invocation of the Workspace long press menu would trigger this issue
   and it would persist until a touch went directly onto the Workspace in a
   location without any children.

Bug 132298752

Change-Id: Id8617baaa1ce59dc84758a7c82049329323b04cc