notif [nfc]: Note a todo-i18n for display names of notif sound files

Toward zulip#277.

These may be tricky to actually handle translating, particularly
given the logic in _ensureInitNotificationSounds that looks at the
filenames of existing files.  They're also in a pretty obscure
corner where I expect very few users see them, so not a priority.

But they (a) are user-facing and (b) use English words ("chime",
"low", "high") to communicate, so are squarely in the category of
strings that should in principle get translated.  Mark them as such.

Author: gnprice

lib/notifications/display.dart

@@ -25,10 +25,11 @@ import '../widgets/theme.dart';
 AndroidNotificationHostApi get _androidHost => ZulipBinding.instance.androidNotificationHost;
 
 enum NotificationSound {
-  // Any new entry here must appear in `keep.xml` too, see #528.
+  // TODO(i18n): translate these file display names
   chime2(resourceName: 'chime2', fileDisplayName: 'Zulip - Low Chime.m4a'),
   chime3(resourceName: 'chime3', fileDisplayName: 'Zulip - Chime.m4a'),
   chime4(resourceName: 'chime4', fileDisplayName: 'Zulip - High Chime.m4a');
+  // Any new entry here must appear in `keep.xml` too, see #528.
 
   const NotificationSound({
     required this.resourceName,