Keep notifications hidden when dreaming

This avoids notifications flickering when tapping to exit the dream,
before the glanceable hub is shown. This is due to a race condition
between the unocclusion logic running which sets StatusBarState back to
KEYGUARD, and the transition from DREAMING->HUB running.

Fixes: 329091482
Flag: ACONFIG com.android.systemui.communal_hub TEAMFOOD
Test: atest SharedNotificationContainerViewModelTest
Change-Id: Ib96c06eeba630c35f048ca1c8479f349f922d11b