Review Mozilla 1642039: Make UpdateManager construction asynchronous

The conflicting file is browser/components/BrowserContentHandler.sys.mjs.

If I understand correctly, this shouldn't be a big deal for us. Opposite, it might be even positive for us: we were moving the homepage retrieval, but upstream moved it for us. So, our patch got smaller.

Anyway, it's better to have a second check. Commits are b9549828 (upstream, 132), which conflicts with 39799007 (ours, 131).