Fix keyguard -> shade_locked scrim transition

A recent fix to not apply the bouncer scrim exposed this bug, where
pulling down the shade over the lockscreen was not correctly setting
the notif scrim.

Remove the hardcoded BLACK value, and align with all other ScrimStates
to use conditional logic for mClipsQsScrim.

Fixes: 259684978
Test: atest ScrimControllerTest
Test: manual pull down shade and use bouncer over lockscreen, in
regular and split shade modes

Change-Id: I057fad9149523e418f909bade90490effdc26eb5