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.
Your code won't work properly for the SystemStorageEvent(http://www.w3.org/2012/sysapps/device-capabilities/#idl-def-SystemStorageEvent)