Improve touch target for the gear beneath a notification

This CL increases the touch target size of the gear beneath
a notification.

It also adds the logic:
- If the gear is displayed and the notification is touched
then the notification slides back over the gear
- If the grey background is touched rather than the gear
  nothing happens

Bug: 22451710
Change-Id: I7c1d8943e066fdd821446c2f90807891d6b74e5c