If the modem is disabled:
* Polling is completely halted.
* Thresholds are disabled.
* The user is allowed to call Setup() or SetupThresholds() to change
the settings, even if the actual polling or thresholds setup isn't
in effect.
When the modem is enabled:
* Polling will be started if there is a existing polling rate.
* Thresholds will be setup based on the existing threshold settings.
Fixes https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/issues/504
45102fcd60