Initialize functions as NONE in legacy handler.

Since adb is being removed from the functions read
from sys.usb.config anyway, it makes more sense to
set it to NONE on boot. This prevents errors with
devices that already have oem functions in the config.

Bug: 73010922
Test: Power test passes
Change-Id: I855ba228dbdd9f8c8a91842a8beb68a789fdf1e9