Send TYPE_NOTIFICATION_STATE_CHANGED events for IMPORTANCE_LOW notifications.

They shouldn't make noise, but because they appear on-screen
we should report them to a11y services to potentially
announce them to the user in some other way.

Bug: 38500162
Test: runtest systemui-notification
Change-Id: I3a022329f54d17c7229a05130dce14c0101ec24a