Scrim was flickering on FPS unlock

After inspecting traces, the scrim state was flickering between
UNLOCKED -> KEYGUARD -> UNLOCKED within a few hundred millis after
unlocking the device. This could produce an inconsistency in state,
where CentralSurfaces thinks keyguardShowing=true but
KeyguardStateController thinks keyguardShowing=false. When the
CentralSurfaces#updateScrimController(), the code was checking both
locations and getting a different answer. Remove mKeyguard from
CentralSurfaces and rely on KeyguardStateController

Fixes: 221732271
Test: atest CentralSurfacesTest and manual locks/unlocks
Change-Id: I6b2ce8e01ac9e61bd31c6928a20303347af79583