Fix device tracking with initial /sync
* Check whether we share an e2e room with user IDs in the 'left'
field of /keys/changes: there's no guarantee we no longer share
any e2e rooms with these users
* Reset everyone's tracking status on an initial sync - just
re-fetching device lists for all users we're currently tracking
isn't good enough since room memberships may have changed.
* Fix typo in test
