Do not remove unfold overlay too early on folding

We were removing the unfold overlay when receiving
'folded' device state update, but this might happen
much earlier than the inner screen fully turns off.

Moved removing of the overlay to onScreenTurnedOff
callback instead, so we remove it only when it is no
longer visible to the user.

Bug: 370682149
Test: fold/unfold manually multiple times
Test: atest UnfoldLightRevealOverlayAnimationTest
Flag: EXEMPT bugfix
Change-Id: I0365aca0e90d3fdc2261703d7c3085e3ccc9641c