title: How to be notified when foreground (top) activity (application) changes
body: <p>I want to write a service for Android platform that is notified when the current foreground activity changes.
  Basically the service should do some tasks only when the top activity changes.
  Is there any way to subscribe  and to be notified when this kind of event occurs ? 
  Or there is no possibility and the service should poll from time to time the list of running activities and to check what is the foreground activity ?Not preferable solution...</p>