Make App Pinning use Stackable list

Summary: This makes setStackble play well in ObjectBox, also tweaks dragging in a stackable box (pinning)

Test Plan: Drag in App Settings, Drag in Maniphest, Workboards

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9413