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.
Rewrite above code as below to save indent
```
if (it->second.capacity == 0)
  continue;

picojson::value unit = picojson::value(picojson::object());
...
```