title: Android informations about floating overlays/elements over other apps
body: <p>I'm looking for some informations about floating overlays
  Not overlays of a mapView but overlays floating over other applications.</p>
  
  <p>For example those applications are doing what I'm looking for :</p>
  
  <p><a href="https://play.google.com/store/apps/details?id=com.pvy.batterybar" rel="noreferrer">https://play.google.com/store/apps/details?id=com.pvy.batterybar</a></p>
  
  <p><a href="https://play.google.com/store/apps/details?id=ds.cpuoverlay" rel="noreferrer">https://play.google.com/store/apps/details?id=ds.cpuoverlay</a></p>
  
  <p>They are displaying elements in front of the screen even if we are not in the application</p>
  
  <p>I tried some searches but I didn't found anything.</p>
  
  <p>Does someone knows where to find some API documentation, a tutorial link, a piece of code ...</p>
  
  <p>Thanks</p>