[flexiglass] Don't change to bouncer scene when panel expansion changes.

In Flexiglass, we don't need to model the appearance of another scene
based on the expansion of a scene; this is what the scene graph is for.

The code here was causing a switch to the bouncer scene when swiping up
on the lockscreen scene while the device was actually unlocked (a state
one can get to by unlocking and unfolded foldable device and then
folding it shut again).

Also made sure that the bugs fixed by ag/20621509 (which added the
legacy version of this code) aren't present in Flexiglass.

Bug: 341445974
Test: manually, see above
Flag: com.android.systemui.scene_container
Change-Id: I07060180ec8bfc2e260e628c787c4d1a06c2a47b