Fix dialog handling.

* Changes
  + VpnSettings:
    + Add reconnect dialog to showDialog() framework for handling screen
      rotation.
    + Dismiss other alert dialogs in onDestroy()
    + Remove onPrepareDialog() from VpnSettings.
    + Remove updateConnectDialog() from VpnProfileActor.
    + Add authentication error dialog.
    + Add unknown server dialog.
  + SecuritySettings:
    + Make cstor dialogs cancelable.
    + Add cancelable listener.
  Patch Set 6:
  + VpnSettings:
    + Disable preferences when connecting/disconnecting.
    + Fix state broadcast when binding a VpnService goes wrong.