Ensure alpha of 0 when GONE

Safety measure to ensure the keyguard alphas are set to 0 when
keyguard transitions to GONE.

Fix: 315345556
Test: atest KeyguardRootViewModelTest
Flag: Flag: ACONFIG com.android.systemui.keyguard_shade_migration_nssl
DEVELOPMENT

Change-Id: Ied9d2d6740a1465042d3c8067669e7be2cb76429