Only show split button for single fullscreen task if taskbar is present in Overview

* Taskbar in overview allows second app to be
selected so user wouldn't be stuck in split
select state

Fixes: 258543259
Test: Tested w/ flag on and off
w/ one and multiple flags
w/ fullscreen and split single focused task

Change-Id: Ie588ad66fde4e012e08d8f5abbe1eef5a1a5db6b