main: Fix syncing animations-enabled

main: Fix syncing animations-enabled

Whether or not animations should be enabled depends on various
factors, some of which may change at runtime. We therefore
track changes, and sync the setting by calling inhibit/uninhibit
as necessary.

Except that we never actually record the new state, so when animations
are disabled, we end up inhibiting them every time
the setting is synced, whoops.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2698>