title: Detect the first time an Activity is opened on this session
body: <p>I need to detect the first time an activity is called every time the user launches the app:</p>
  
  <ul>
  <li>App is launched</li>
  <li>Activity X is called for the first time
  <ul>
  <li>Do something</li>
  </ul></li>
  <li>Activity Y, Z are called many times</li>
  <li>Activity X is called again many times
  <ul>
  <li>Do nothing</li>
  </ul></li>
  </ul>
  
  <p>... so "do something" has to be called only if the app was closed (or killed) and then be launched again.</p>