workspaceAnimation: Get time outside of the animation-complete callback

workspaceAnimation: Get time outside of the animation-complete callback

We want to use the current event time for activating the workspace, and
this time is only available when calling global.get_current_time() or
Clutter.get_current_event_time() from the context of an event handler.

So instead of trying to get that time when the animation has finished
from the onComplete() handler, get it before and store it as a variable
to use in the onComplete() handler later.

This fixes an error message when switching workspaces using the swipe
gesture where MetaWorkspace complains about the 0-timestamp we pass it.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2233>