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

scripts/gen_build_prop.py

@@ -19,9 +19,12 @@
 import argparse
 import contextlib
 import json
+import os
 import subprocess
 import sys
 
+TEST_KEY_DIR = "build/make/target/product/security"
+
 def get_build_variant(product_config):
   if product_config["Eng"]:
     return "eng"