Add vendor to list of supported partitions in gen_build_prop.py

The kati built vendor.img will still use the kati built vendor
build.prop file, but the soong built vendor.img will use an
autogenerated `build_prop` module for vendor

This CL should be a noop for now

Bug: 374371755
Test: diff in contents for aosp_cf_x86_64_phone https://paste.googleplex.com/5766413037076480
Summary
- timestamp change (expected)
- ro.hwui.use_vulkan changes from "" to false, but I think this is
  functionally equivalent
Test: presubmits

Change-Id: I10bc5b6e8d48b86fa70d5ab0722fee658b1a224a