Fix state when connection toggle is tapped twice (#16)

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.

android/app/src/main/kotlin/net/defined/mobile_nebula/NebulaVpnService.kt

@@ -50,7 +50,12 @@ class NebulaVpnService : VpnService() {
         val id = intent?.getStringExtra("id")
 
         if (running) {
+            // if the UI triggers this twice, check if we are already running the requested site. if not, return an error.
+            // otherwise, just ignore the request since we handled it the first time.
+            if (site!!.id != id) {
                 announceExit(id, "Trying to run nebula but it is already running")
+            }
+
             //TODO: can we signal failure?
             return super.onStartCommand(intent, flags, startId)
         }