title: How can I tell if my Application is resumed from background?
body: <p>I want to lock my application when it goes in background and 
  when it resumes I want to display my own lock screen. The lock screen is an Activity of my application.</p>
  
  <p>After successfully entring password the user can see the resumed Activity 
  else he can't.</p>
  
  <p>How can I do this?</p>