Change LiskDAO::generatePHID to be public

Summary: Ref T6822. There are a bunch of places where we call $something->generatePHID(...) externally (outside of the class). Therefore, these methods need to be public.

Test Plan: I wouldn't expect increasing method visibility to break anything.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6822

Differential Revision: https://secure.phabricator.com/D11363