title: How can I use the paid version of my app as a "key" to the free version?
body: <p>Let's say for example that I have some Android app that does X.  The free version has ads or basic features.  I want to have a paid version that removes the ads and adds extra features.</p>
  
  <p>How can I use the paid app as a "license key" to unlock the features in the free app?</p>
  
  <p>So the user would install the free app, then install the paid app to get the extra features, but they would still run the free app (which would now be unlocked).  What's the best approach to doing this?</p>