Prevent flickering during dream transition

The shade expansion changes during the dream transition from 0->1 due to
status bar state changing. This causes the keyguard root alpha to be set
to 1 temporarily during the transition. Prevent this by ignoring shade
expansion changes while transitioning to the dream.

Bug: 361796575
Test: atest KeyguardRootViewModelTest
Flag: EXEMPT bugfix
Change-Id: I82f30df731698295ffea5f65c9fa3c285042f251