Adds the screen off animation!

The way this works is essentially:
- We tell PowerManager we want to handle the screen off animation, like we do for the Lockscreen -> AOD transition
- KeyguardViewMediator holds off on showing the activity lock screen, so that the underlying activity remains visible during the animation
- The scrim animation plays normally, which results in the 'reveal' effect in reverse
- We force the AOD views to full dozing, so that we can simply translate them in after a delay once the scrim has un-revealed the screen contents

(demo video: https://drive.google.com/file/d/1cRjg22wpQuvn9FPugJSkIF-cEOfM2qXi/view?usp=sharing)

Test: unlock/lock with no security
Test: unlock/lock with PIN/password lock set
Test: same, with face auth
Test: use all security methods with/without notifications showing on lock screen
Bug: 169693662
Change-Id: Ied66f11fc9820f2810cf931b1960041218d60b63