NSSL: Smooth transition from shade->lockscreen

The current animation is rather abrupt. Have it fade in with the rest
of lockscreen.

Also, remove flatmaps from bounds and max notification
calculations. This was causing a slight delay in updating those
values. Use a straightforward combine instead.

To ensure a smooth transition, make sure the legacy finger tracking
stops properly.

Fixes: 310993802
Test: atest SharedNotificationContainerViewModelTest
Flag: ACONFIG com.android.systemui.keyguard_shade_migration_nssl
DEVELOPMENT

Change-Id: Icff993a5622290f98ee699220a4010eb1fbba015