[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.

Test: added unit and integration tests for the new code that exposes the
unfoldProgress
Test: manually verified that gently folding up the device correctly
slides the elements of the split shade into the center (true for the
left-hand QS side and status bar, but needs more work for the
notifications side, even though the code applies the translation it's
not happening yet)
Flag: ACONFIG com.android.systemui.scene_container DEVELOPMENT
Bug: 330483283

Change-Id: I91db7922e824ee106faa96709b069f5b862aa7de