[WPE] prefers-reduced-motion is inverted, animations disabled by default

https://bugs.webkit.org/show_bug.cgi?id=325212

Reviewed by Patrick Griffis.

SystemSettingsManagerProxy::reducedMotion() negates the reduced-motion
setting, so the documented default of false reports that the user
prefers reduced motion. Every site that honours the
prefers-reduced-motion media query then disables its animations.

The negation is left over from enableAnimations(), which this function
replaced in 446267f9f445 ("[WPE][GTK] Improvements to platform media
queries (prefers reduced motion, high contrast, and dark theme)"). There
it was correct, because the value being read was the opposite setting,
WPE_SETTING_DISABLE_ANIMATIONS. Both the function and the setting
changed polarity together, so the negation has to go.

Canonical link: https://commits.webkit.org/322794@main