title: How to handle SYSTEM_ALERT_WINDOW permission not being auto-granted on some pre-Marshmallow devices
body: <p>I've been getting reports of some Xiaomi devices (e.g. Mi 2, running API level 21) not showing overlays.  My app targets API 23.</p>
  
  <p>There are <a href="http://forum.xda-developers.com/xiaomi-mi-3/help/link-bubble-playing-nice-t2857607" rel="noreferrer">several</a> <a href="http://en.miui.com/thread-30931-1-1.html" rel="noreferrer">posts</a> out there regarding this.  It seems that MIUI devices do not enable this permission at install time (unlike other pre-Marshmallow devices).</p>
  
  <p>Unfortunately, <code>Settings.canDrawOverlays()</code> only works on Android 23+.</p>
  
  <ol>
  <li>What is the correct way to check whether this permission has not yet been enabled pre-Marshmallow?</li>
  <li>Is there an Intent to take the user to the relevant MUIU settings page? Maybe: <code>new Intent("android.settings.action.MANAGE_OVERLAY_PERMISSION", packageName)</code> but I have no means to test this.</li>
  </ol>