Fix lock screen flash when transitioning from hub to edit mode

Keyguard is hidden when the alternate bouncer is showing over the hub,
adjust the start value of the transition accordingly.

Bug: 325108777
Test: atest AlternateBouncerToGoneTransitionViewModelTest (WIP)
Flag: ACONFIG com.android.systemui.communal_hub TEAMFOOD
Change-Id: I60c41b175af5c3c69f620457f6c9d1daa349dd44