overview: Sync grabs after running startup animation

In the nested session the startup animation sometimes fails, weirdly that always happens to me when running a freshly built gnome-shell the first time. The reason it fails is that mutter fails to aquire a pointer grab from the xserver, XIGrabDevice() is unsuccessful.

A simple workaround for this race condition in the xserver is to just grab the devices a bit later, that is after the startups animation instead of before it.

This was also tested with disabled animations, and seems to work just as well in that case.