[flexiglass] Unfold transition support in the shade scene.

When a foldable is fully unfolded and then the user begins to fold it
up, there's a subtle animation that happens to elements across the
screen. Left-hand side elements move to the right and right-hand side
elements move to the left, seeming to gently float towards the fold
hinge.

This CL adds that for Flexiglass, only for the (split) shade scene and
only for the NSSL, which didn't have that until now.

Test: updated unit and integration tests for the new code that exposes the
unfoldTranslationX
Test: manually verified that gently folding up the device correctly
slides the elements of the split shade into the center (both left-hand
side elements and the NSSL on the right)
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Bug: 330483283

Change-Id: I10a9f941110e484dc5a81322558611b422a1a9d8