Fix black frame when unfold animation starts

The dark scrim was added with a value of 0f (that means, completely black), and then updated in the background thread with a value of 1f (completely transparent).

This was ok when unfolding (as the overlay needed to be black at first), but not when folding (as it needs to be transparent at first)

When the background thread was slow, this was causing one completely black frame at the beginning of the animation.

Bug: 261560634
Test: start unfold animation repeatedly
Change-Id: I98e17d0204cb4d75a85d3c31c6a6faa8e58466f0