title: How to get android lock screen wallpaper?
body: <p>I use the code below to retrieve the android lock screen wallpaper on an android 8.1 phone:</p>
  
  <pre><code>WallpaperManager manager = WallpaperManager.getInstance(getActivity());
  ParcelFileDescriptor pfd = manager.getWallpaperFile(WallpaperManager.FLAG_LOCK);
  if (pfd == null) // pfd is always null for FLAG_LOCK, why?
      return;
  Bitmap lockScreenWallpaper = BitmapFactory.decodeFileDescriptor(pfd.getFileDescriptor());
  // ...
  </code></pre>
  
  <p>I have granted the <code>READ_EXTERNAL_STORAGE</code> permission and set a lock screen wallpaper beforehand.</p>
  
  <p>I run the demo on a real phone, and found the <code>pfd</code> is always null for <code>FLAG_LOCK</code>, so I cannot get the lock screen wallpaper. Please help fix the problem, thanks.</p>