Tapping the ticker again activates the notification.

(Previous behavior: the tap would pass through to the clock,
which would open the shade. Only sometimes it wouldn't,
because you'd have hit the left-hand-side of the ticker
where there's no clock underneath. So this fixes that too.)

Additionally, tapping the ticker will now immediately
dismiss the ticker.

Bug: 3365129
Change-Id: Ic641184c518b18d799a560c8da6b4c5844c912de