Use a policy-aware query in PhabricatorSearchSelectController

Summary: Ref T603. This didn't impact policies anyway, but using PhabricatorObjectQuery is far simpler and more general.

Test Plan: Used "Attach" dialog to find mocks, tasks, and revisions by "Dxx", "Mxx", etc.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D7195