Fix keyguard status bar showing upon entering dream

When transitioning from launcher or lockscreen to dreaming, keyguard
status bar flickers upon entering dream.
Fix by updating the status bar alpha with the value in the transition
view model.

Bug: b/372563526
Test: atest KeyguardStatusBarViewControllerTest
Test: from lockscreen/launcher, verify status bar is hidden upon entering dream
Flag: EXEMPT bugfix
Change-Id: I982dfaa07a55853ab7ce92c64cd0141db3f4da90