title: UsbManager.getDeviceList() returns empty
body: <p>I want to use the <code>UsbManager</code> API by wrote following code:</p>
  
  <p>In the Activity code:</p>
  
  <pre><code>        UsbManager manager = (UsbManager) getSystemService(Context.USB_SERVICE);
  
          HashMap&lt;String, UsbDevice&gt; deviceList = manager.getDeviceList();
  
          Log.d(TAG, "size =" + deviceList.size());
  </code></pre>
  
  <p>In the Manifest:</p>
  
  <pre><code>&lt;uses-sdk
      android:minSdkVersion="8"
      android:targetSdkVersion="17" /&gt;
  
      &lt;permission android:name="android.hardware.usb.host" &gt;
  
  &lt;/permission&gt;
  
  &lt;uses-feature android:name="android.hardware.usb.host" android:required="true" /&gt;
  </code></pre>
  
  <p>I also followed <a href="https://stackoverflow.com/questions/11183792/android-usb-host-and-hidden-devices">this question</a>, checked my Galaxy SII 9100G. It does have <code>/system/etc/permissions/android.hardware.usb.host.xml</code>, and the right permissions.</p>
  
  <p>But <code>manager.getDeviceList()</code> always return zero, when I tried flash-driver and mouse.</p>
  
  <p>Can anyone give me any suggestion? </p>