AttentionSetUtil: Add missing license header

Change-Id: I014b661a0b0e72bc690485370763cf983b8225e3
Signed-off-by: Edwin Kempin <ekempin@google.com>

java/com/google/gerrit/server/util/AttentionSetUtil.java

@@ -1,3 +1,17 @@
+// Copyright (C) 2020 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.server.util;
 
 import com.google.common.collect.ImmutableSet;