effects/private: Fix ExpoLayout reflowing in Desktop Grid while dragging

This fixes makes ExpoLayout independent of its position on a screen,
which in turn stops it from randomly reflowing while dragging a virtual
desktop across desktop grid. It also removes an unwanted animation at
the end of activation of desktop grid: desktops move in place and then
windows proceed to adjust themselves resulting in a double animation.

FIXED-IN: 5.25.5 5.26