title: Android: how to quit an activity without closing it when click on back button?
body: <p>I have a subActivity that can be open from my mainActivity.</p>
  
  <p>For some reasons, when the user clicks on the back button go back to my mainActivity, I want my subActivity to remain open in background in order to be able to come back for later.</p>
  
  <p>Questions:</p>
  
  <ul>
  <li>how to avoid to close the subActivity when the user clicks back ?</li>
  <li>how to come back to the mainActivity without restarting it ?</li>
  <li>how to come back later to my opened activity without re-creating it completely ? (just want to bring it to front)</li>
  </ul>
  
  <p>Thanks !</p>