Bug 1919363 - Only show one app menu "new window" item in permanent private browsing. r=mconley

We also update the browser_private_browsing_window.js test.
The previous test was limited because it was referring to non-existent
"appmenu_newNavigator" and "appmenu_newPrivateWindow".

Differential Revision: https://phabricator.services.mozilla.com/D222507