Officially deprecate skin autodiscovery

Corresponding change in master (core skins no longer use autodiscovery
there, but it's too scary of a change to backport here at last minute):
I6fc30f97d4b35820c132c4892abf6d2c86f9482e.

Bug: 65748
Change-Id: If9125ef587ce11f0d69f12eacd00ae7236139798