Make sure scrims will fade from black

Scrims might be light if the wallpaper is light, but when
turning on the screen, we should always fade from black,
otherwise the display will bright up before dimming down.

Test: pulse with light wallpaper and AOD on/off
Test: press power with AOD on/off
Test: atest ScrimControllerTest
Fixes: 138097741
Change-Id: Icc1ce276c839bce248ab9362a5d8cf4905201e0e