SystemUI: Make clock truly invisible

* When the clock is set to left and then hidden, the notification
  icons, which would appear on the left first transition to the
  right to make place for the clock then finally snap into place
* The same happens when the clock was set to show right and then
  is hidden: The battery, wifi, etc. icons do leave space for the
  (not displayed) clock and then snap to the right
* This patch fixes the behavior

Change-Id: Ic68b1001becb91da6f1aeaca95a7312b36e1d2a1