Scrims - Don't blend with bouncer when shade is collapsing

In the case of having transitioned back to KEYGUARD state, never blend
with the bouncer scrims unless the bouncer is actively in
transit. This prevents the scrim from flashing after collapsing the
shade over lockscreen.

Fixes: 248405303
Test: atest ScrimControllerTest
Test: manually display shade, lockscreen, bouncer and change dark modes
Change-Id: I645115abeb09822a99508b9d9f1fc974b7ce31be