SystemUI: locked tasks should have a valid view

If the view doesn't exist, the card won't be able to receive touch
events. We need those touch events to have a bounce-back animation
when trying to swipe away a locked card.

Change-Id: If7b3c5e5094d2752926eb306dd54264da0ceaa3e
Signed-off-by: Joey Rizzoli <joey@lineageos.org>