usb: gadget: Add device attribute to determine gadget state

Android frameworks (UsbDeviceManager) relies on gadget state exported
through device attributes.  This CL adds the device attribute to export
USB gadget state.

Change-Id: Id0391810d75b58c579610fbec6e37ab22f28886d
Signed-off-by: Badhri Jagan Sridharan <Badhri@google.com>