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.
nit comment
```
if(storages_.empty())
  return false;

std::map<int, SysInfoDeviceStorageUnit>::iterator it;
for(it = storages_.begin(); it != storages_.end(); ++it) {
```