Call ensureActivitiesVisibleLocked when setShowWhenLocked

If an activity is behind lockscreen and then just calls
setShowWhenLocked(true), nothing will happen, as it won't trigger AM to
update the visibilities of the activities. We need to make sure to
actually update the visibilities in this case.

Test: ran go/wm-smoke tests

Change-Id: I4fb384a3d5ef8427c2eee32986f1dcbd07514825