Prevent a sync request from removing a user's busy presence status (#12213)

In trying to use the MSC3026 busy presence status, the user's status
would be set back to 'online' next time they synced. This change makes
it so that syncing does not affect a user's presence status if it
is currently set to 'busy': it must be removed through the presence
API.

The MSC defers to implementations on the behaviour of busy presence,
so this ought to remain compatible with the MSC.