STL improve large appbar demo experience

Simplifying overscroll logic in the demo by removing logic based on what
has already been consumed by descendants.

Test: Manually tested on a device, videos in b/378470603#comment15
Bug: 378470603
Flag: com.android.systemui.scene_container
Change-Id: I6dd489c41a6e102c5a0b90298764379162362be6