Add "device ready" flag to standalone dialog pages

Summary: Fixes T5541. Standalone dialog pages, including the high-security auth page, should all work fine on mobile.

Test Plan: {F173598}

Reviewers: chad, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T5541

Differential Revision: https://secure.phabricator.com/D9799