[Notif] Limit shown events to manager.show

We have a slight over-fire of show events due to bindNotification being
called multiple times on density or user config changes. To be extra
careful, limit the show events to only get called with
perhapsShowBlockingHelper.

Bug:74609669
Test: Through adb logcat -b events + thru config changes
Change-Id: I0719052869e53ff1cba5030579daf512f324e99c