Fix a type issue with FormView juggling

Summary:
When you click the pencil icon in the Maniphest task list, we currently fatal:

  Argument 1 passed to PhabricatorCustomFieldList::appendFieldsToForm() must be an instance of AphrontFormView, instance of PHUIFormLayoutView given, called in /core/lib/phabricator/src/applications/maniphest/controller/ManiphestTaskEditController.php on line 576 and defined

This is because we build an `AphrontFormView` noramlly, but a `PHUIFormLayoutView` for dialogs, since they don't take a full form (they render their own form tag).

Instead, always build an `AphrontFormView` and just pull the `PHUIFormLayoutView` out of it when we're ready to put it in a dialog. This means `$form` is always the same type of object, and is generally better and makes more sense.

Test Plan: Clicked pencil edit icon in Maniphest task list.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran, carl

Differential Revision: https://secure.phabricator.com/D8324