Fade in the FolderIcon stroke after Folder animates closed.

When the Folder animates closed, it hides itself the same
time as the FolderIcon becomes visible. Because the Folder
does not have the white stroke outline, there is a visual
jump between the closed Folder and the FolderIcon.

This change is a subtle and easy step towards reducing
the visual jump.

Bug: 35064148
Change-Id: I8aeeed873e7144499d19f3b01c85bfa8a792097f