Use the slow spatial spec in OffsetOverscrollEffect

This CL makes the OffsetOverscrollEffect use the slow spatial spec
instead of the default one, because it feels much better. In the future,
we should probably select the slow/fast/default depending on the
overscrollable size, but at the moment it's fine to use this one as the
main use case is the flexiglass SceneContainer.

Bug: 378470603
Test: Manual, feels better
Flag: com.android.systemui.scene_container
Change-Id: I806a00ef0bafae77574ffc424e0a7954220446ec