API feedback: setSuppressableBubble/setSuppressBubble

Apps can't actually suppress the bubble, that is only
done by the system so that method is @hide.

Did a similar thing for setSuppressNotification for
consistency.

Test: atest NotificationManagerTest (see CTS CL)
Bug: 184204395
Change-Id: Ia17453507daf5bba4408bd6b386efa225c3051dd