Add error code in vpn connectivity broadcast.

* Changes
  + Add VpnConnectingError.java.
  + Broadcast the error returned by daemons.
  + Add error codes to VpnManager.java.
  + Add error code to VpnManager.broadcastConnectivity().
  Patch Set 4:
  + Replace VPN_UP with VPN_STATUS in response to ip-up-vpn changes.
  + Make VpnServiceBinder a foreground service so that it won't be
    interrupted by the system.
  Patch Set 5:
  + Remove the support of returning 0 from daemon and restart socket in
    AndroidServiceProxy.