Add dark scrim from a background thread during unfold

This attaches the black scrim in the background. All changes to that view have to go though the background handler.

Bug: 254924424
Test: current unfold tests should work unchanged && manually verified things are as expected with a perfetto trace
Change-Id: I224d9084b6f20230810faa6cc6d6b8bd6b46f518
1 file changed