Fix scrim busyness regression

Scrim opacity on the keyguard should vary according to how many
notifications are visible.
A busy lock screen has a more opaque scrim, and an empty lock screen
has a more transparent scrim.

Test: atest packages/SystemUI/tests/src/com/android/systemui/statusbar/phone/ScrimControllerTest.java
Test: swipe away or receive notifications on the keyguard
Change-Id: I672b5e1a82d65dd326b40069d61223c3c0161493