Make sure background tasks are not used for shutdown cleanup

network.cache.shutdown_purge_in_background_task is a pref that makes privacy.sanitize.sanitizeOnShutdown use background tasks rather than doing it in the Firefox process.

The reason is that doing it in the process slows down the shutdown.

In my first analysis, I thought that the outcome was that when background tasks aren't available (our case) the cleaning is never done... Now I'm not sure, because in the diff revision there are comments about this. But rather than having a direct logic, upstream relies on an error condition for running it on the main process... maybe. It's kinda hard to follow.

At the moment, this pref is true by default only on Windows. However, I'd flip it to false, so that if anyone flips privacy.sanitize.sanitizeOnShutdown (it's false by default for us), at least they get the cleanup in the main process.