Fix keyguard flicker on QR code launch

For some reason this was very reproducible only with
this activity. Ensure alpha won't show during transition.

Fixes: 366022169
Test: atest KeyguardRootViewModelTest
Flag: com.android.systemui.migrate_clocks_to_blueprint
Change-Id: Iec51a50555512996783756830864a5793b90c974