vendor: fix ro.adb.secure for vendor-building devices

* This ensures the prop is set correctly for devices
  that build a vendor image by placing the prop in
  /system/etc/prop.default, rather than /vendor/default.prop
  so it'll be in a prop file that is run for mangle_default_prop
  in build/make/tools/post_process_props.py, therby
  setting persist.sys.usb.config correctly to "none".

Change-Id: I4027541cf1eae9dd967636efe35de9578922b725