usb: gadget: Accessory:Migrate to USB_FUNCTION API

This patch adds support to use Android accessory
gadget function through the
DECLARE_USB_FUNCTION_INIT interface.

Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
Change-Id: Ib352752d5bc905fa1df9049b53eabf1294930db7