Prevent double-load of Conpherence threads

Summary: We may execute the Conpherence behavior before the initial device change, in which case we'll get a desktop -> desktop device event. This currently causes us to double-load the thread list. Instead, don't do anything if the device is the same as our current understanding of device state.

Test Plan: No double thread list in profiler.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D5525