chore: license headers

Author: MichaelVerdon

packages/firebase_database/firebase_database/android/src/main/kotlin/io/flutter/plugins/firebase/database/TransactionHandler.kt

@@ -1,3 +1,9 @@
+/*
+ * Copyright 2022, the Chromium project authors.  Please see the AUTHORS file
+ * for details. All rights reserved. Use of this source code is governed by a
+ * BSD-style license that can be found in the LICENSE file.
+ */
+
 package io.flutter.plugins.firebase.database
 
 import android.util.Log