Revert "[effects/slidingpopups] Start the animation on only windowAdded or windowClosed"

This reverts commit 9d4c8fda.

Unlike other effects this wasn't using hide/show as a hack for not
having a created/destroyed signal but because the window is internally
hidden when it's in auto-hide mode despite being still mapped.

BUG: 426686


(cherry picked from commit 095cdcd3)