[Dual Shade] Refactor + show the status bar clock on QS shade.

BONUS: Remove `isShadeLayout` from `ShadeHeaderViewModel`.

Fix: 397197000
Test: Added unit tests.
Test: Existing unit tests still pass.
Test: Manually by opening the quick settings shade on both narrow and
 wide screens and verifying the status bar clock is shown in both cases.
Flag: com.android.systemui.scene_container
Change-Id: I57b812607508121d6fe59b42e2b3bc490bd27b0d