title: Determine where a gmail add-on is running
body: <p>I'm developing an add-on for gmail and I'd like it to show different texts depending on whether it's running on the browser or on the android app, but I don't know if it's possible.</p>
  
  <p>I've tried using <code>HtmlService.getUserAgent()</code> and the deprecated <code>UiApp.getUserAgent()</code>, but both of them return <code>null</code>; and I haven't found any other possibilities on the documentation.</p>
  
  <p>Can anyone help on this? Thanks!</p>