msglist [nfc]: Make MessageListPageState public

Coming up, this State will store a handle onto the compose box (a
`GlobalKey<ComposeBoxController>`), which the message action sheet
will use to conduct a quote-and-reply interaction. In order for the
action sheet to access that handle, it needs to know about
MessageListPageState.

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -25,10 +25,10 @@ class MessageListPage extends StatefulWidget {
   final Narrow narrow;
 
   @override
-  State<MessageListPage> createState() => _MessageListPageState();
+  State<MessageListPage> createState() => MessageListPageState();
 }
 
-class _MessageListPageState extends State<MessageListPage> {
+class MessageListPageState extends State<MessageListPage> {
   @override
   Widget build(BuildContext context) {
     return Scaffold(