Emit CallAdded only after the list of calls has been updated and notified

We should force the emission of the PropertiesChanged signal in the modem object as soon as a call object is added in the internal tracking list, and only then emit CallAdded. In that way, clients would receive first the updated call list and only then the CallAdded signal, and they would be able to find the matching call in the already updated internal list.

Reported in the mailing list: https://lists.freedesktop.org/archives/modemmanager-devel/2018-July/006494.html