Enable `privacy.resistFingerprinting.spoofOsInUserAgentHeader` by default when Tor Browser is set to safest mode

When Tor Browser is set to safest mode, and JavaScript is disabled, spoofing the user agent should effectively spoof the users OS.

Putting all safest mode Tor Browser users in the same bucket will increase the anonymity set, as opposed to maintaining different buckets of varying sizes which lowers the anonymity set for obscure operating systems.