Soong: Return `dev-keys` by default if no cert had set

Change-Id: I99c315be21d7b1f3155cedb8bf4d267c9cb3a910
diff --git a/scripts/gen_build_prop.py b/scripts/gen_build_prop.py
index fcf8b41..2dba94f 100644
--- a/scripts/gen_build_prop.py
+++ b/scripts/gen_build_prop.py
@@ -43,10 +43,10 @@
   default_cert = product_config.get("DefaultAppCertificate", "")
   if "ROM_BUILDTYPE" in product_config:
     return "release-keys"
-  elif default_cert == "":
-    return "dev-keys"
   elif default_cert == os.path.join(TEST_KEY_DIR, "testKey"):
     return "test-keys"
+  else:
+    return "dev-keys"
 
 def override_config(config):
   if "PRODUCT_BUILD_PROP_OVERRIDES" in config: