[flutter_plugin_tools] Check licenses in Kotlin (#4373)

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.

script/tool/lib/src/license_check_command.dart

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