1772862 - Make `--backgroundtask backgroundupdate` work when default profile has langpacks installed

Status: NEW

(Toolkit :: Application Update, enhancement, P3)

Description

[Nick Alexander :nalexander [he/him]]

The initial background update landing does not schedule the background update task when the default profile has langpacks installed. This ticket tracks making that work.

This work isn't particularly high priority because only about 5% of Firefox release users have langpacks installed: see this STMO query.

In a February 2022 conversation, :mixedpuppy and :nalexander concluded that it would be possible to duplicate the existing in-product langpack/application update coordination in the background updater. It should be fairly straight-forward to teach the background update:

1. to determine langpacks installed into the default profile
2. to fetch any updates available
3. to stage/sideload said langpacks into the default profile directory so that they are in place for the restart after application update.
   Most of these APIs already exist in the AddonManager.jsm.