Bug 41668: Port some updater patches to Base Browser

Changes that will need to remain when actually cherry-picking to
base-browser, because they contain name fixes, for example.
Or, for UpdateService.jsm, they need to stay because they make the patch
more understandable.

Bug 41668: Port some updater patches to Base Browser

Remove Tor Browser-only additions to make the updater patches usable
with Base Browser.
This commit should be reverted, and become a commit on its own, to
further adapt the updater to Tor Browser needs.

This commit was once part of "Bug 4234: Use the Firefox Update Process
for Tor Browser.".
However, some parts of it were not needed for Base Browser and some
derivative browsers.
Therefore, we extracted from that commit the parts for Tor Browser
legacy, and we add them back to the patch set with this commit.