Add modemmanager driver

Uses the dbus go module to talk to ModemManager. Gets locations by polling the GetLocation MM method and supports loading A-GPS / XTRA data from a file (/var/cache/gnss-share/xtra.bin) with the MM API.

Tested on the Pinephone and Oneplus 6. Everything works fine on short tests, but disappearing and appearing modems are not supported yet and this probably causes things to break eventually, at least on the Pinephone.

Does not enable A-GPS MSA or MSB, as this does not seem to improve performance on the Pinephone, and is not supported on Oneplus 6.

This is progress towards solving #10.