title: Confirm and discard button in the android preferences activity
body: <p>I'm working on the preference activity. The matter is about the possibility of add a button that allow to confirm, and eventually (that would be nice) a discard button in order to discard the changes applied at the preferences. </p>
  
  <p>Let me explain better.</p>
  
  <p>As far as I've seen it is possible using the common techniques that i've found in the tutorial to set different preferences using checkboxes and so on. The normal use case that involve preferences require that the user performs selections and after that click the BACK button in order to return to the old view. </p>
  
  <p>However in the usability test i've done, it seems that this step is not always straightforward for all the users, and moreover a lot of them are not sure about the fact that the changes on the preferences are comfirmed.</p>
  
  <p>Now we arrive at the question. is it possible somehow to have in the preference view selections (in particular one just composed by a group of checkboxes) to have an OK and eventually CANCEL button?</p>