screenShield: Fix showing shield without animation

Commit 24e631ffe2 changed the shield animation to use translation
instead of position.

However once the shield is raised, only an animation will lower it
again, which means the shield is missing when it's supposed to be
shown without animation (for example after an idle blank).

Fix this by resetting the translation-y property in that case.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/927

js/ui/screenShield.js

@@ -1025,6 +1025,7 @@ var ScreenShield = class {
                 },
             });
         } else {
+            this._lockScreenGroup.translation_y = 0;
             this._lockScreenGroup.fixed_position_set = false;
             this._lockScreenShown({ fadeToBlack, animateFade: false });
         }