Only allow hub mode to show when keyguard is visible

Since the hub mode SceneTransitionLayout is in the notification shade
window, it's currently accessible when the notification shade is open,
even if the device is unlocked or an activity is open.

This CL hides the SceneTransitionLayout when keyguard is not visible.
Future CLs will also allow the SceneTransitionLayout to show on top of
the dream.

Bug: 308813166
Flag: ACONFIG com.android.systemui.communal_hub DEVELOPMENT
Test: manually verified on device
Change-Id: I80b8a4020016d476c758e3a78e4c78c6d89b89e2