Only show TV PiP Notification once

The user will no longer see a new notification pop up when the
notification content is changed (e.g. when expanding/collapsing the
PiP), but the NotificationListenerService will still receive the updated
content.

Bug: 235474366
Test: manual - toggling expanded PiP does not show a new notification +
NotificationHandler still receives an onNotificationPosted call

Change-Id: I5cf7e467c695499bb5b5dfc6cecddc10694b8e02