Fade keyguard when entering glanceable hub

Temporary transition for teamfood purposes, full animation to come
later.

Also requires NSSL and keyguard bottom area refactor flags to take
effect:
adb shell device_config override systemui com.android.systemui.keyguard_shade_migration_nssl true
adb shell device_config override systemui com.android.systemui.keyguard_bottom_area_refactor true

Bug: 315205222
Test: atest KeyguardRootViewModelTest
Flag: ACONFIG com.android.systemui.communal_hub DEVELOPMENT
Change-Id: I44ae8865cbf3e970cdd7750659ebae62f1130f13