Add TODOs for registerReceiver calls without flag

PiperOrigin-RevId: 461165173
(cherry picked from commit 5a163765)

library/ui/src/main/java/com/google/android/exoplayer2/ui/PlayerNotificationManager.java

@@ -1162,6 +1162,7 @@ public class PlayerNotificationManager {
     Notification notification = builder.build();
     notificationManager.notify(notificationId, notification);
     if (!isNotificationStarted) {
+      // TODO(b/197817693): Explicitly indicate whether the receiver should be exported.
       context.registerReceiver(notificationBroadcastReceiver, intentFilter);
     }
     if (notificationListener != null) {