Swipe to unlock - Fix alpha continuity

Many transitions should start from the last value used in the UI
layer, such as alpha, translationX, translationY. In the case of swipe
to unlock, as the user is swiping up, elements are fading out until a
certain point is reached which then triggers a LOCKSCREEN->GONE
transition. This transition should begin from the current alpha value
rather than some other arbitrary point.

Fixes: 322198222
Test: atest KeyguardRootViewModelTest
SharedNotificationContainerViewModelTest
LockscreenToGoneTransitionViewModelTest
Test: manual - slowly swipe to unlock
Test: manual - fling swipe to unlock
Flag: ACONFIG com.android.systemui.keyguard_shade_migration_nssl
DEVELOPMENT
Flag: ACONFIG com.android.systemui.keyguard_bottom_area_refactor
STAGING

Change-Id: Ida64adfbde26654548f80426cb4050b4aedfa52e