[flexiglass] Remove an unflagged change from StackScrollAlgorithm

StackScrollAlgorithm#updateAlphaState received an unflagged change in I709b0e1fc188b3cd0b6554837ac757af049c816f, which hits reads a flexi field from AmbientState and hits an SceneContainerFlag.isUnexpectedlyInLegacyMode assertion. This is a forward fix to restore the legacy codepath.


Change-Id: Idf13b559a5154bf86e27be3aa458d880e3e85125
Fixes: 370452903
Test: open app shortcuts while being in split shade
Flag: com.android.systemui.scene_container