msglist [nfc]: Use fetched getter when reading

Generally this is helpful because it means that viewing references
to the field will highlight specifically the places that set it.

Here it's also helpful because we're about to replace the field
with an enum shared across several getters.

Author: gnprice

lib/model/message_list.dart

@@ -697,7 +697,7 @@ class MessageListView with ChangeNotifier, _MessageSequence {
     if (!narrow.containsMessage(message) || !_messageVisible(message)) {
       return;
     }
-    if (!_fetched) {
+    if (!fetched) {
       // TODO mitigate this fetch/event race: save message to add to list later
       return;
     }