Add an explicit delay before taking a screenshot in A11yMenu.

Even when we request to hide the UI before taking a screenshot it was
still appearing in ~1/30 screenshots. By adding a slight delay I haven't
been able to capture it in a screenshot again.

Uses the same flag as before, but I've rolled back this flag to 0%
rollout in order to re-land.

Bug: 292020123
Test: Take 100 screenshots as fast as I can using A11yMenu;
      No screenshot contains A11yMenu UI.
Change-Id: I4e0a07f73082181254c46e8c35b7f8a38ab7158b