Implement .sysprop Api Checker

This introduces a new message for storing API informations of sysprop
modules. sysprop_api_checker ensures that APIs of sysprop_library won't
break backwards compatibility.

Bug: 131637873
Test: m && sysprop_test
Change-Id: I4e2cfb2227e5ab1b9beda9cada1292ce89fba130
diff --git a/include/ApiChecker.h b/include/ApiChecker.h
new file mode 100644
index 0000000..cac2499
--- /dev/null
+++ b/include/ApiChecker.h
@@ -0,0 +1,25 @@
+/*
+ * Copyright (C) 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <android-base/result.h>
+#include <string>
+#include "sysprop.pb.h"
+
+android::base::Result<void> CompareApis(
+    const sysprop::SyspropLibraryApis& latest,
+    const sysprop::SyspropLibraryApis& current);