ANDROID: usb: gadget: mtp/ptp: Migrate functions to the USB_FUNCTION interface

This patch adds support to use mtp/ptp gadget functions
through the DECLARE_USB_FUNCTION_INIT interface.

enabling USB_CONFIGFS_F_MTP config compiles f_mtp.c
thereby providing support for MTP gadget

enabling USB_CONFIGFS_F_PTP config compiles f_ptp.c
thereby providing support for PTP gadget

Signed-off-by: Badhri Jagan Sridharan <badhri@google.com>
Change-Id: I38d7b570e8886d155ef10cd2c839b2232dcb3158