Add a `setEncType` method to AphrontDialogView.

Summary: Fixes a Conpherence fatal when going to /conpherence/update/1

Test Plan: Successfully rendered the edit form and saved it.

Reviewers: epriestley, btrahan

Reviewed By: btrahan

CC: aran, Korvin

Maniphest Tasks: T2402

Differential Revision: https://secure.phabricator.com/D4634