[Flexiglass] Fade Notifications during Lockscreen -> Dream transitions

Dream is not a Keyguard state anymore, because it is presented as a
separate Scene. Update the transition mappings, so the animated alphas
will work again.

Fixes: 438459091
Test: go from LS to Dream -> check the fade animations
Test: go from Dream to LS -> check the fade animations
Flag: com.android.systemui.scene_container
Change-Id: I2f091bfe5904ec6a5e32c614ccff60be920968c6