[Flexiglass] Add tests for closing the shade with back button

Add asserts for Back action where Swipe.Up actions is tested
as they have the same role.
Add handling of ToSplitShade transition to Back action to replicate
Swipe.Up behavior.

Bug: 430910634
Test: atest ShadeUserActionsViewModelTest
Flag: com.android.systemui.scene_container
Change-Id: Iaf4b6dcfb55d63c7211ecb6a70a2896b1353a3ad