DeskClock: DigitalAppWidget: Don't listen to non working intents

* These two are currently not called anyway:
  * SCREEN_ON: would have to be explicitly registered, currently not
    even implicitly called
  * DATE_CHANGED: Not sure what does the trick instead, but it's
    currently not called and we get the correct date anyway (tested
    by setting time to 23:59 and observing the widget)

Change-Id: I99ae8653ad712d8ac35d105ab85dd1ce57360961