modem-manager: Set up notifier callbacks to reset MM location properties

Watch the ModemLocation properties we use ('enabled', 'signals-location' and 'gps-refresh-rate') and reset them to the values we want if they are changed by other ModemManager clients.

In order to avoid reset ping-pong, all clients using the ModemLocation interface in ModemManager should obey the following protocol:

1. Maintain the wanted state of ModemLocation properties in local vars.
2. When enabling sources, always preserve other enabled sources.
3. Setup notifier callbacks for the necessary properties. If the properties are downgraded (sources or signaling disabled, refresh rate lowered), reset them to the wanted values. Never downgrade properties in the notifier callbacks.
4. When disabling sources, turn off everything we don't want. Other clients can turn these properties on in their notifier callbacks.