Smooth burn-in transitions

GONE->AOD transitions are easily interruptible by a power button push
to keep the phone on, or to prevent unlocking. This adds a smoother
transition from AOD->LOCKSCREEN which is next in the sequence. Rather
than rely on a preset starting point, it begins from the current
y-translation value and interpolates to the final resting point.

Also, adds the ability for animation flows to return both a value and
the transition state. The transition state is frequently used in view
models to know which transition is truly active.

Fixes: 322197793
Test: manual - interrupt GONE->AOD
Test: atest com.android.systemui.keyguard
Flag: ACONFIG com.android.systemui.keyguard_shade_migration_nssl
DEVELOPMENT

Change-Id: If9501500796bee9d421a291bf555077e8eefcc14