[Flexiglass] Don't emit the same values from Set<HeadsUpRowKey> flows

Only emit from the Set<HeadsUpRowKey> flows if there is a change, to make
the binder's job easier when it is diffing for previous keys to generate
animation events.

Bug: 400691495
Flag: com.android.systemui.scene_container
Test: observe HUN intro/outro animations

Change-Id: I75f7cae9fd87b5d869d17976bf37ba06b6da26a3