status/system: Add context to a11y label

"Lock Screen" can refer to the lock screen component or the action
of locking the screen. Add context to clarify that the button
refers to the latter.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5852

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2485>

js/ui/status/system.js

@@ -232,7 +232,7 @@ class LockItem extends QuickSettingsItem {
             style_class: 'icon-button',
             can_focus: true,
             icon_name: 'system-lock-screen-symbolic',
-            accessible_name: _('Lock Screen'),
+            accessible_name: C_('action', 'Lock Screen'),
         });
 
         this._systemActions.bind_property('can-lock-screen',