Fix method visibility for PhabricatorApplicationSearchEngine methods
Summary: Ref T6822.
Test Plan: Visual inspection. These methods are only called from within the PhabricatorApplicationSearchEngine class.
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: Korvin, epriestley
Maniphest Tasks: T6822
Differential Revision: https://secure.phabricator.com/D11242