Fix PhabricatorActionView::getAvailableIcons

Summary: getAvailableIcons parses icon(s).json, which has recently been pluralized in D5890.

Test Plan: /uiexample/view/PhabricatorActionListExample/

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5902