[Dual Shade] When transitioning between the shades, collapse at start.

Previously, the collapse only started at the end of the transition.

Fix: 418724318
Test: Tested manually by switching between the shades.
Test: Updated unit tests.
Flag: com.android.systemui.scene_container
Change-Id: I07a8547c654ee49b9bbea665de8fd66c1718cdd8