Use getPhobjectClassConstant() to access class constants

Summary: Ref T9494. Depends on D14216. Remove 10 copies of this code.

Test Plan: Ran arc unit --everything, browsed Config > Modules, clicked around Herald / etc.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9494

Differential Revision: https://secure.phabricator.com/D14217