Fix method visibility for PhabricatorPolicyAwareQuery subclasses

Summary: Ref T6822.

Test Plan:
grep for the following:

• ->willFilterPage(
• ->loadPage(
• ->didFilterPage(
• ->getReversePaging(
• ->didFilterPage(
• ->willExecute(
• ->nextPage(

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: hach-que, Korvin, epriestley

Maniphest Tasks: T6822

Differential Revision: https://secure.phabricator.com/D11367