Bug 1718444 - Don't show the update-available prompt if we aren't going to...

Bug 1718444 - Don't show the update-available prompt if we aren't going to download the update r=nalexander,application-update-reviewers

This patch fixes an issue where it is possible to show the update-available prompt to the user for an update that will not ultimately be downloaded. This can result in many unnecessary update-available prompts. The issue is that `AUS.downloadUpdate` makes some checks to ensure that it doesn't download updates if, for example, that exact update has already been downloaded. But the update-available prompt is shown before `AUS.downloadUpdate` is called.


Depends on D126163

Differential Revision: https://phabricator.services.mozilla.com/D126164