The ModemManager1.Sim.PreferredNetworks property contains the preferred
networks (and access technologies, if available) configured to the
SIM card.
This commit implements preferred networks reading with AT+CPOL.
In preparation for the multi-SIM setup, we need a way to tell whether
a given SIM card is active or not in the system.
On systems with one single SIM slot, the available SIM card will
always be active.
On Multi-SIM Single-Standby setups we may have multiple SIM slots with
multiple SIM cards, but only one of them will be active at any given
time.
On Multi-SIM Multi-Standby setups we may have multiple SIM slots with
multiple SIM cards that may be active at the same time. E.g. the QMI
protocol allows up to 5 different active SIM cards (primary,
secondary, tertiary...).
