unlockDialog: Use unique unlock-dialog style class

Allows styling the unlock dialog elements individually from the GDM
login dialog.

https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1421

data/theme/gnome-shell-sass/widgets/_login-dialog.scss

@@ -5,7 +5,8 @@
   max-width: 23em;
 }
 
-.login-dialog {
+.login-dialog,
+.unlock-dialog {
   //reset
   border: none;
   background-color: transparent;

js/ui/unlockDialog.js

@@ -475,7 +475,7 @@ var UnlockDialog = GObject.registerClass({
     _init(parentActor) {
         super._init({
             accessible_role: Atk.Role.WINDOW,
-            style_class: 'login-dialog',
+            style_class: 'unlock-dialog',
             visible: false,
             reactive: true,
         });