Restore user-accessible lights-out control.

Current system area gestures:

       tap - no-op (bug 3114340)
  swipe up - open system panel
swipe down - lights out
long press - lights out (bug 3134973)

The enter/exit animations have been tweaked to reinforce
this, particularly the swipe down for lights out. (Swiping
up to exit lights out will work, as will any kind of tap in
the lights-out "curtain".)

Change-Id: Ie027d7a0e86a402d06a8a368a5a43050a6bb9e58