Create account-indexed extension point

Add a new extension point so that plugins can be notified when an
account has been indexed.

Change-Id: I5cba1be9df06b70d870540722d099c3d8397c351

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/events/AccountIndexedListener.java

@@ -0,0 +1,29 @@
+// Copyright (C) 2017 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.
+
+package com.google.gerrit.extensions.events;
+
+import com.google.gerrit.extensions.annotations.ExtensionPoint;
+
+/**
+ * Notified whenever an account is indexed
+ */
+@ExtensionPoint
+public interface AccountIndexedListener {
+/**
+ * Invoked when an account is indexed
+ * @param id of the account
+ */
+  void onAccountIndexed(int id);
+}