ModemManager should report network strength changes more quickly

This is related to and based on an issue discovered in https://source.puri.sm/Librem5/ModemManager/issues/28 . In that ticket, attempting to place or receive a call when one had poor 3G/2G coverage would fail, however because the user didn't see that the network switched from 4G to 3G within a reasonable amount of time, there wasn't a good way for the user to know that the failure was related to signal strength.

Apparently the dbus-monitor picks up updates from MM very quickly, so we just need MM to report when it drops from 4G to 3G/2G for phone calls more quickly so the user can see the strength of the connection.