Fix unlock transition when docked stack is visible

- Keep surface of docked divider when lockscreen is showing, like
all other apps, so we don't run into synchronization issues when
unlocking. We do this by setting policy visibily to false, but
keep clientVisibility=true
- Update docked stack visibility when starting unlock sequence to
visible.

Bug: 27154882
Change-Id: I6def5c18831e23579fcfc904133749f149eaf376