[flutter_plugin_tools] Check licenses in Kotlin

The license check overlooked Kotlin, since it's not currently widely
used in our repositories.

Also adds the missing license to one Kotlin file, from an example that
was (likely accidentally) re-generated using Kotlin instead of Java.

packages/shared_preferences/shared_preferences/example/android/app/src/main/kotlin/io/flutter/plugins/example/MainActivity.kt

@@ -1,3 +1,7 @@
+// Copyright 2013 The Flutter Authors. 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.example
 
 import io.flutter.embedding.android.FlutterActivity

script/tool/CHANGELOG.md

@@ -6,6 +6,7 @@
   federated packages that have been done in such a way that they will pass in
   CI, but fail once the change is landed and published.
 - `publish-check` now validates that there is an `AUTHORS` file.
+- `license-check` now validates Kotlin files.
 
 ## 0.7.1
 

script/tool/lib/src/license_check_command.dart

@@ -16,6 +16,7 @@ const Set<String> _codeFileExtensions = <String>{
   '.h',
   '.html',
   '.java',
+  '.kt',
   '.m',
   '.mm',
   '.swift',

script/tool/test/license_check_command_test.dart

@@ -66,6 +66,7 @@ void main() {
         'html': true,
         'java': true,
         'json': false,
+        'kt': true,
         'm': true,
         'md': false,
         'mm': true,