GameSpace: Properly filter duplicated notification

* Some apps like Telegram will repost unread messages along with new messages sent by someone others.

  Previously, we were just comparing text between notifications. It's not that accurate since people
  would spam same message text at different time which will be treated as a duplicated message.

  Take advantage of Notification.when, which records the time when notification gets posted,
  and won't change on a repost.

Signed-off-by: cjh1249131356 <cjh1249131356@gmail.com>
Signed-off-by: Pranav Vashi <neobuddy89@gmail.com>