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

libs/WindowManager/Shell/src/com/android/wm/shell/pip/tv/TvPipNotificationController.java

@@ -114,6 +114,7 @@ public class TvPipNotificationController {
                 .setOngoing(true)
                 .setCategory(Notification.CATEGORY_SYSTEM)
                 .setShowWhen(true)
+                .setOnlyAlertOnce(true)
                 .setSmallIcon(R.drawable.pip_icon)
                 .setAllowSystemGeneratedContextualActions(false)
                 .setContentIntent(createPendingIntent(context, ACTION_FULLSCREEN))