Previously when `stopVpn()` was called, it was possible for the network
change callback to fire while we were in the middle of shutting down.
This commit unregisters the network change callback before telling
Nebula to shutdown.
Fixes#15. When tapping the toggle in rapid succession,
`NebulaVpnService.onStartCommand` is called twice, in serial. This
method includes logic to show an error to the user if they somehow
attempt to connect to a service while already connected.
However, this method of showing an error message (calling
`announceExit`) sends a signal to `MainActivity` telling it the service
has exited, and that it should set the UI state to "Disconnected." It
does not actually disconnect the service at this point, resulting in a
state mismatch in which you cannot actually disconnect the service.
The solution in this commit is to remove this signalling and simply
return out of `onStartCommand` to avoid processing the start request
twice if the site is already running.
Nate Brown
John Maguire