Fix method visibility for AphrontFormControl methods

Summary: Ref T6822.

Test Plan: Visual inspection. These methods are only called from within the AphrontFormControl class.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6822

Differential Revision: https://secure.phabricator.com/D11249