title: canRead() and canWrite() both return false
body: <p>I'm somehow not able to read or write files.  I'm using the following to get a filename.  It's probably something obvious to most, but my Java is a bit weak.</p>
  
  <pre><code>private String fileName(int itemNum) {
      final String DIR_PREFIX = "/MyDailySelfiesDir";
      final String FILENAME_PREFIX = "/MyDailySelfieFile_";
      File dir_0 = Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES);
      String dirName = dir_0.getAbsolutePath() + DIR_PREFIX;
      File dir = new File(dirName);
      dir.mkdirs();
      String name = dirName + FILENAME_PREFIX + itemNum + ".jpg";
      File temp = new File(name);
      Log.i(TAG, name + " can read? " + temp.canRead() + " can write? " + temp.canWrite());
      return name;
  }
  </code></pre>
  
  <p>AndroidManifest.xml looks like this, in part:</p>
  
  <pre><code>&lt;uses-sdk
      android:minSdkVersion="18"
      android:targetSdkVersion="19" /&gt;
  &lt;uses-feature android:name="android.hardware.camera" /&gt;
  &lt;uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" /&gt;
  </code></pre>
  
  <p>I'm in Android Studio, emulating a Galaxy Nexus 5, API 18.</p>