Fade NSSL with brightness slider

This fix only applies for the migrate clocks flag, and explicitly
does not work for scene container. Basically, a temporary shim.

Fixes: 346298008
Test: atest KeyguardRepositoryImplTest
Test: manual - use brightness slider in split shade
Flag: com.android.systemui.migrate_clocks_to_blueprint
Change-Id: Ie80e45dea9900dcdd5f443284e5bc3633eed6f99