title: listFiles() returns null on Android 6.0 emulator
body: <p>I want to read jpeg files from sdcard on Android 6.0 emulator, but file list returns null. The sample code can work on my phone:</p>
  
  <pre><code>            String sdcard = Environment.getExternalStorageDirectory().toString();
              File sdcard_dir = new File(sdcard);
              if (sdcard_dir.isDirectory()) {
                  File[] fileNames = sdcard_dir.listFiles(new FileFilter() {
                      @Override
                      public boolean accept(File pathname) {
                          return pathname.toString().endsWith(".jpg") ? true : false;
                      }
                  });
              }
  </code></pre>
  
  <p>fileNames = null!</p>
  
  <p>I can use adb shell to list image files on emulator:</p>
  
  <p><a href="https://i.stack.imgur.com/8g8Y6.png"><img src="https://i.stack.imgur.com/8g8Y6.png" alt="enter image description here"></a></p>
  
  <p>The permission has been added to AndroidManifest.xml:</p>
  
  <pre><code>&lt;uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" /&gt;
  </code></pre>
  
  <p>Anything else I can do?</p>
  
  <p>I have to use the emulator because my device cannot be upgraded to Android 6.0 so far, and I want to test some new APIs.</p>