notif: Rename stream to channel in displayed notifications

Fixes: #630

Author: Khader-1

lib/notifications/display.dart

@@ -96,7 +96,7 @@ class NotificationDisplayManager {
       FcmMessageStreamRecipient(:var streamName?, :var topic) =>
         '$streamName > $topic',
       FcmMessageStreamRecipient(:var topic) =>
-        '(unknown stream) > $topic', // TODO get stream name from data
+        '(unknown channel) > $topic', // TODO get stream name from data
       FcmMessageDmRecipient(:var allRecipientIds) when allRecipientIds.length > 2 =>
         zulipLocalizations.notifGroupDmConversationLabel(
           data.senderFullName, allRecipientIds.length - 2), // TODO use others' names, from data

test/notifications/display_test.dart

@@ -166,7 +166,7 @@ void main() {
       final stream = eg.stream();
       final message = eg.streamMessage(stream: stream);
       await checkNotifications(async, messageFcmMessage(message, streamName: null),
-        expectedTitle: '(unknown stream) > ${message.subject}',
+        expectedTitle: '(unknown channel) > ${message.subject}',
         expectedTagComponent: 'stream:${message.streamId}:${message.subject}');
     }));