mm-iface-modem: Always check properties for disabling signal polling

MMBroadbandModemQmi only disables signal polling once it successfully sets the indication up. To do that, it sets the properties on MMIfaceModem. This, however, doesn't actually disable polling, as the property values are only checked once in get_private when Private structure hasn't been created yet. Once that happens, only the cached values are ever checked, and there's nothing that would update them.

Since there's only one place where these properties are checked, get rid of cached bools in Private structure and instead read the property values directly in periodic_signal_check_step, letting QMI modems actually disable polling at runtime.