[telepathy-mission-control] Only consider the connman 'online' state to be connected

In connman, 'ready' means that the interface is configured but not
tested. It moves to 'online' when a test succeeds and the interface is
known to be working.

More importantly, when the default route/network changes, the connman
state will move from online -> ready -> online. IM accounts should be
reconnected in that situation.

By monitoring only for the 'online' state, we will consider 'ready' to
be the same as offline and not attempt connections to any IM accounts.
It's unclear if that will have any unwanted effects in practice.