ANDROID: GKI: USB: Resolve ABI diff for usb_gadget and usb_gadget_ops

Also, adds function usb_gsi_ep_op and resolves ABI diff for
struct usb_ep_ops by adding new member gsi_ep_ops. Also
resolves ABI diff for struct usb_request by adding member udc_priv.

Test: build
Bug: 153676200
Change-Id: I04a59f7a2670fb80afa5822b79ea488bc94abdfe
(cherry picked from commit 62523500)
[hridya: EXPORT_SYMBOL -> EXPORT_SYMBOL_GPL, cherry-picked only the ABI
diff, added some null pointer checks and documentation]
[mkbestas]: Reapply after this got missed in between the merges somehow
Signed-off-by: Hridya Valsaraju <hridya@google.com>