store [nfc]: Mark unawaited future from registerNotificationToken

The called method returns a not so useful future that is used for
testing.

See also:
  zulip#325 (comment)

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/model/store.dart

@@ -726,7 +726,7 @@ class UpdateMachine {
     updateMachine.poll();
     // TODO do registerNotificationToken before registerQueue:
     //   https://github.com/zulip/zulip-flutter/pull/325#discussion_r1365982807
-    updateMachine.registerNotificationToken();
+    unawaited(updateMachine.registerNotificationToken());
     return updateMachine;
   }