DeskClock: DigitalAppWidget: Work with cities again

* Not explicitly adding the receiver for at least our own broadcasts
  results in warnings but no actual actions being taken
* By explicitly listening, the digital clock widget now will show
  world clocks again if you choose them in the app itself
* Test included checking if any of the other implicitly registered
  actions are received anyway - we don't need to add them manually then

Change-Id: I5c40c9a329892630db1ec3a8ec36f9f80a26736a