improve Ofono plugin

This fixes several crashes (including #248 (closed)) and logic errors (such CallsOfonoOrigin's construct implementation clearing its modem field which caused the origin to be repeatedly re-added, triggering an assertion in the record store), bringing the Ofono backend to rough parity with the MM backend. Tested on a PinePhone.

I had intended to upstream changes in this direction about 7 months ago but better late than never.