[flexiglass] Ensure shade header height matches the status bar height.

Bonus: Simplify the transition spec to the QuickSettings scene.

Fix: 396405667
Bug: 413766503
Bug: 408486251
Bug: 409594391
Test: Manually by expanding the shade and observing the height matches
 that of the status bar, on both a folded and unfolded device.
Flag: com.android.systemui.scene_container
Change-Id: Id83813b8d58a556bb289d22d3f6429c2c59b20d3