ANDROID: usb: gadget: configfs: 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.

Bug: 68755607
Bug: 120441124
Change-Id: Id0391810d75b58c579610fbec6e37ab22f28886d
[badhri: Migrate to using udc uevents from upstream sysfs.]
Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
[AmitP: Folded following android-4.9 commit changes into this patch
        Parts of e45c769fa7af ("ANDROID: usb: gadget: cleanup: fix unused variable and function warnings")
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>