A11yMenu: also delay before locking the screen.

Internally the screen lock action is handled in SysUI by taking
a screenshot and then fading that to black, so the delay allows
us to hide the UI before that screenshot is taken.

Uses the same flag `a11y_menu_hide_before_taking_action`
that was used for the "take screenshot" delay because this flag
has not yet rolled out to the larger population.


Bug: 309557993
Test: Enable the flag, lock the screen, observe A11yMenu UI
      is hidden from lockscreen fade animation.
Change-Id: I55cd656bdfa10670dd4eb6db943625ae00793ba8