[Status bar] Delay showing the status bar during dream transition.

Bug: 273314977
Test: manual: trigger lockscreen to dream transition -> verify that the
status bar icons in the top left don't flicker visible for a few frames
(see video attached to bug)
Test: atest CollapsedStatusBarFragmentTest
CollapsedStatusBarViewModelTest

Change-Id: I3474a5c7f6f485e8d26ffc92eac31a9b08901d51