title: How to set system app is not disabled?
body: <p>How can I set a system app (my custom system app) to be NOT DISABLED? 
  Just like the Launcher or Settings that the "Disable" button is not enabled. So the user can't disable my system app.</p>
  
  <p>Below is the Launcher's effect:</p>
  
  <p><img src="https://i.stack.imgur.com/JkBKI.png" alt="Launcher&#39;s effect"></p>