Dual tone shade implementation

Splitting behindScrim into two separate scrims: qs and notificatins.
QsScrim role is solely to show dark background behind quick settings.
NotificationsScrim serves as past behindScrim, that is takes fullscreen
when on keyguard and draws notifications background.
Bottom of the quick settings panel determines where qsScrim ends and
notificationsScrim begins.

Fixes: 183953523
Test: atest ScrimControllerTest
Change-Id: I86897f23072281745cef23ef8eed51e06b21818b