title: DownloadManager fails with ERROR_UNKNOWN on API 17
body: <p>I'm trying to use android's own DownloadManager and it works perfectly on API 18+ but the same code fails (STATUS_FAILED) with reason ERROR_UNKNOWN almost as soon as I enqueue it on API 17 phones. here's my code</p>
  
  <pre><code>Context context = MyApplication.getSharedContext();
  DownloadManager.Request request = new DownloadManager.Request(Uri.parse(url));
  request.setNotificationVisibility(DownloadManager.Request.VISIBILITY_VISIBLE).setTitle(notiTitle).
  setVisibleInDownloadsUi(false);
  File file = new File(Environment.getExternalStorageDirectory().getAbsolutePath()+File.separator+fileName);
  request.setDestinationUri(Uri.fromFile(file));
  DownloadManager manager = (DownloadManager) context.getSystemService(Context.DOWNLOAD_SERVICE);
  manager.enqueue(request) ;
  </code></pre>