UrfDeviceOfono: toggle the oFono Online property on the modem for block state.

Signed-off-by: Mathieu Trudel-Lapierre <mathieu.trudel-lapierre@canonical.com>
UrfArbitrator: make sure toggling a killswitch type-wide blocked affects oFono devices

Signed-off-by: Mathieu Trudel-Lapierre <mathieu.trudel-lapierre@canonical.com>