title: mkdir() fails with SD card although there is permission
body: <p>I try to create a folder in sdcard</p>
  
  <pre><code>File folder = new File(Environment.getExternalStorageDirectory().getPath() + File.separator + "folder");
  Log.d(TAG, "FOLDER :" +folder);
  folder.mkdir();
  </code></pre>
  
  <p>mkdir always return false. I added permission to manifest file</p>
  
  <pre><code>&lt;uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/&gt;
  </code></pre>
  
  <p>I can create folder with <code>adb</code> tool.</p>
  
  <p>Phone is Nexus 5 Android 6.0.1</p>
  
  <p>what is wrong with code ?</p>