[flexiglass] Enable toggling legacy SysUI on/off.

To manually toggle legacy SysUI on/off, top 5 times in quick succession
on the device camera (top center). That particular spot was chosen
because it's the least likely one to have actual UI tap targets
underneath that the toggle would interfere with.

Note: due to b/290984671, the tap target after initial unlock moves
directly below the camera.

Fix: 290927217
Test: manually verified by tapping on the camera to toggle the UI on and
off.

Change-Id: I0d7d905a31842a8450436472a0a94d9e58bc2fda