Listen for network disconnect.

VPN used to just watch the interface, but that is insufficient.  There
is no promise that the interface will go down when we're done with it.
Now that wifi stays on in scan-only mode despite user turning it off
it seems that the interface is left up, even in AP mode.

Now listening for ConnectivityService broadcast that the network we were on
has disconnected and tearing down the VPN then or when the interface
goes away.

bug:8550083
Change-Id: Icf414497bc55bead69de04e91f39f90ac2e6578a