iface-modem: allow low-power request while enabled

We now enable low-power mode request if we are enabled or even connected. In this case, an explicit modem disable operation is run, so that we don't end up with an enabled modem in low-power mode.

This new "low-power+disable" single operation makes the two "disable+low-power" operation combo that was run until now much much quicker, because the modem unregisters from the network and disconnects PDNs in the same network interaction.