Conditionally use Unix epoch time for build incremental

Incremental version is used in various places to invalidate caches.
Setting BUILD_NUMBER directly causes unnecessary rebuilds due to
environment variable changes, so simply set it to Unix epoch unless
BUILD_NUMBER is explicitly set.

Change-Id: Id590df48ae1b73b63039f185644103d66a4bbbb3

scripts/gen_build_prop.py

@@ -102,6 +102,9 @@ def parse_args():
 
   config["MinusDevice"] = config["DeviceName"]
 
+  if config["BuildNumber"].startswith("eng."):
+    config["BuildNumber"] = config["DateUtc"]
+
   override_config(config)
 
   append_additional_system_props(args)