Implement new lights-out mode in system bar.

When an app requests fullscreen display, "shadows" of the
visible system bar UI elements are drawn in their place. The
user can still interact with these elements by poking the
shadows; the widgets will temporarily shine through, long
enough for the user interaction.

Known issues:
 - if the notification panel is up for too long, the shadow
   will not be re-enabled on the notification area (need to
   route all hide/show requests through the
   shadowcontroller)
 - status bar hide/show animations have been temporarily
   turned off to make this work correctly; they'll be put
   back later

Bug: 3203171