[fenix] For https://github.com/mozilla-mobile/fenix/issues/22851 add wallpapers feature flag

app/src/main/java/org/mozilla/fenix/FeatureFlags.kt

@@ -101,4 +101,9 @@ object FeatureFlags {
      * Enables showing the option to clear site data.
      */
     val showClearSiteData = Config.channel.isNightlyOrDebug
+
+    /**
+     * Enables showing the wallpaper functionality.
+     */
+    const val showWallpapers = false
 }