Change unfold background color based on split screen state

After this cl, task background will have the same color of the split
divider when the fold/unfold animation happens while in splitscreen.

For all other cases, the behaviour is the same as before.

Before this cl, the task background and split divider had different
colors that didn't match.

Bug: 278071265
Test: 1. Folded while in split screen
2. checked the colors match
3. went to single app
4. checked the color was the original one

Change-Id: I6915ea0c249a2c1de15f59ed01f8290e0b8b02b8