title: sharing auth token between android apps
body: <p>I have a requirement to share a common presistent login (auth token) between 2 or more android apps.  The trick is that either app need no be installed for the other to work.  They are independent of each other.  </p>
  
  <p>So before an app logs in it asks the question "is there possibly another friendly app out there that can give me (or has stored somewhere) a token that I can use?"</p>
  
  <p>Obviously there are various ways (and issues) that I can use to approach this: </p>
  
  <ul>
  <li>using a shared service (requires secondary installation, which app installs it?)</li>
  <li>using a shared content provider (requires secondary installation, which app installs it?)</li>
  <li>using a file on the system (if the file exists / may not be accessible on every device)</li>
  <li>using shared preferences (i believe that on some android version global shared prefs are not possible)</li>
  <li>using an ordered broadcast (to wake up another possible app and ask it)</li>
  </ul>
  
  <p>What do the stackoverflow folks think is the best approach that is simple but also robust?</p>