Clip keyguard root view as the shade expands

This should prevent keyguard content from being visible under or above
the shade. Future flexiglass work should fix this completely with
correct z-ordering, but the existing implementation with shared scrims
can result in overlapping rendering.

Test: manual, expand/collpase shade in all orientations and with
splitshade
Test: atest KeyguardRepositoryImplTest KeyguardRootViewModelTest
ScrimControllerTest
Flag: ACONFIG com.android.systemui.keyguard_shade_migration_nssl
DEVELOPMENT

Change-Id: If5e7b40b0ca7565947bf1c695c14fb08995e547e