Flight mode async

This change includes a number of changes to urfkill in order to fix a few longstanding critical flight-mode bugs, namely:

https://bugs.launchpad.net/ubuntu/+source/urfkill/+bug/1321627
https://bugs.launchpad.net/ubuntu/+source/urfkill/+bug/1339794
https://bugs.launchpad.net/ubuntu/+source/urfkill/+bug/1354716
https://bugs.launchpad.net/ubuntu/+source/urfkill/+bug/1354716


Why did you not leave this as-is? I think it's the proper place for the object path computation to happen strictly at the generic device, rather than reimplementing the exact same logic in multiple places. From the actual devices, you only need to pass the device index, which *is* specific to the devices (also since we put ofono devices and hybris devices in a separate range...)