Decouple key fetcher; Update ECMG API

protos/public/BUILD

@@ -0,0 +1,36 @@
+################################################################################
+# 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.
+################################################################################
+
+# Protocol buffer definitions for Widevine media cas packager sdk.
+
+package(default_visibility = ["//visibility:public"])
+
+filegroup(
+    name = "binary_release_files",
+    srcs = glob(["**"]),
+)
+
+proto_library(
+    name = "media_cas_encryption_proto",
+    srcs = ["media_cas_encryption.proto"],
+)
+
+cc_proto_library(
+    name = "media_cas_encryption_cc_proto",
+    deps = [":media_cas_encryption_proto"],
+)
+
+proto_library(
+    name = "media_cas_proto",
+    srcs = ["media_cas.proto"],
+)
+
+cc_proto_library(
+    name = "media_cas_cc_proto",
+    deps = [":media_cas_proto"],
+)