Transitions - Legacy dream -> lockscreen flicker

It briefly shows lockscreen content before hiding and fading in
nicely. This is due to the possible ordering of occlusion/dreaming
events so make sure to start the transition as soon as either changes.

Fixes: 267600416
Test: atest KeyguardTransitionScenariosTest

Change-Id: I337d0189af72e052b805a213d0faa8f6b528811d