[fenix] Issue https://github.com/mozilla-mobile/fenix/issues/5233: Added call to doesPrivateBrowsingPinnedShortcutExist function to determine whether the "add private browing shortcut" item is visible in the settings menu.

app/src/main/java/org/mozilla/fenix/settings/SettingsFragment.kt

@@ -132,6 +132,10 @@ class SettingsFragment : PreferenceFragmentCompat(), AccountObserver {
             }
         }
 
+        findPreference<Preference>(getPreferenceKey(R.string.pref_key_add_private_browsing_shortcut))?.apply {
+            isVisible = !PrivateShortcutCreateManager.doesPrivateBrowsingPinnedShortcutExist(requireContext())
+        }
+
         setupPreferences()
 
         updateAccountUIState(context!!, requireComponents.backgroundServices.accountManager.accountProfile())