Make interim WAN cache key deactivation logic broader

Make interim WAN cache key deactivation logic broader

* This is how it was before d5aa846d8, which accidentally changed
  the behavior. The WAN cache call in Setup.php was too coupled to
  the ChronologyProtector cookies. It made the assumption that DB
  positions and cookies are stored even when there are no replica
  databases setup (which changed in that patch).
* Check the "UseDC" cookie instead, which is already set in exactly
  the cases this logic should apply.

Bug: T185328
Change-Id: I4dd2a6ca6cdead0052d59be86e6030dbe01f141b