Fix build error for gen_build_prop.py

Provided undefined constants and imports.

Bug: 346214958
Test: Define "PRODUCT_DEFAULT_DEV_CERTIFICATE := \
                 build/make/target/product/security/testkey" and
      m aosp_cf_system_x86_64
Change-Id: Ic65f059fe87a997a9be695ebf13dfde2d2c6f82c