ensure that Windows default browser agent is omitted

From #33534 (closed): Firefox 76 added a controversial agent (system service?) on Windows that reports info about the default browser to Mozilla even if Firefox is never opened. We should set default-browser-agent.enabled to false and/or ensure that the agent is not included in Tor Browser.

https://bugzilla.mozilla.org/show_bug.cgi?id=1624047
"Default browser agent not being packaged"