[flexiglass] use LSToGoneTransitionViewModel for keyguardAlpha during LS -> GONE

With flexiglass on, using the shade/QS alpha for keyguard when on Gone scene was causing the stack to flicker when going from LS -> Gone scene, as well as during the first shade invocation after going from LS -> Gone. This CL switches us over to the alpha provided by LockscreenToGoneTransitionViewModel and only uses shade/QS alpha when shade/QS are actually occluding keyguard.

Bug: 364887368
Test: Manually verified keyguard alpha during LS -> Gone transition
Flag: com.android.systemui.scene_container
Change-Id: I837156b5e39cfc512ec7564824683a5acb6fc98b