[flexiglass] Fix dual shade locked shade notif expansion

KTF's handling of STL idle state wasn't accounting for overlays yet

Bug: 383157288
Test: verified KTF correctly switches to UNDEFINED state when in locked shade via logging
Test: added unit tests
Flag: com.android.systemui.scene_container
Change-Id: Id759deb5140223722193f42d7edbee3aed3516ed