Fix dreaming to GH transition in v2

We previously kept the dream alpha at 100% during the transition, since
the GH would fade in on top of the dream. However, since GH now has a
transparent background, we must first transition the dream to 0% alpha
before showing GH.

Bug: 394496191
Test: transitioning to/from dream to GH and lockscreen
Flag: com.android.systemui.glanceable_hub_v2
Change-Id: Ic477b53677141c81088d11ce6b85bf3641daca60