Implement the detection of USB Flash devices on IVI.

BUG=XWALK-2040

This patch implement the feature of detecting USB Flash devices on ivi, as well as getting its "capacity" and "isRemovable" attributes.
```
if (!dev) {
  usleep(1000);
  continue;
}

int dev_id = udev_device_get_devnum(dev);
...
```