title: UsbManager returns null when getAccessoryList is called
body: <p>I'm developing an app in which android-powered device acts as USB-Device(not USB-Host). So I need to find the host that my android device is connected to. Here is my code:</p>
  
  <pre><code>btnRefresh.setOnClickListener(new View.OnClickListener() {
              @Override
              public void onClick(View v) {
                  //UsbAccessory[] usbAccessory = new UsbAccessory[1];
                  mUsbManager = (UsbManager)getSystemService(Context.USB_SERVICE);
                  usbAccessory = mUsbManager.getAccessoryList();
                  List&lt;String&gt; accessoryListTemp= new ArrayList&lt;String&gt;();
  
                  if (usbAccessory != null) {
                      if (usbAccessory.length &gt; 0) {
                          String hhhh = usbAccessory[0].getModel();
                          accessoryListTemp.add(hhhh);
                      }
                  }
                  //accessoryListTemp.add("hamed test item");
                  ArrayAdapter&lt;String&gt; arrayAdapterTemp = new ArrayAdapter&lt;String&gt;(getApplicationContext(),
                         android.R.layout.simple_spinner_item, accessoryListTemp);
                  arrayAdapterTemp.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
  
                  spinnerHosts.setAdapter(arrayAdapterTemp);
              }
          });
  </code></pre>
  
  <p>I cannot figure out why I got null in accessoryListTemp variable. It probably means that there is no device found, but actually my device is connected and the other device finds it.
  To be specific I'm connecting two android phone via USB and one of them is host and the other is device, but the device doesn't recognize host.</p>