Refactor and cleanup codes. No functional changes.

common/python/drm_service_certificate.clif

@@ -0,0 +1,18 @@
+################################################################################
+# Copyright 2018 Google LLC.
+#
+# This software is licensed under the terms defined in the Widevine Master
+# License Agreement. For a copy of this agreement, please contact
+# widevine-licensing@google.com.
+################################################################################
+
+from "util/task/python/clif.h" import *
+from "common/python/drm_root_certificate.h" import *
+from "common/drm_service_certificate.h":
+  namespace `widevine`:
+    staticmethods from `DrmServiceCertificate`:
+      def AddDrmServiceCertificate(
+              root_certificate: DrmRootCertificate,
+              service_certificate: bytes,
+              service_private_key: bytes,
+              service_private_key_passphrase: bytes) -> Status