[Dual Shade] Fix ScrimStartableTest for Dual Shade on.

This ensures these unit tests do not fail when Dual Shade is the default
shade experience in scene container.

BONUS: Remove the unused isOnKeyguard precondition, as only
isDeviceEntered is used to determine the keyguard state.

Bug: 376411622
Test: Updated unit tests.
Flag: com.android.systemui.scene_container
Change-Id: I1fbbbdeba086ddcafb0a944516d1009174325b70