Consider setting "Prevent accessibility services from accessing your browser" by default

Someone asked on the blog why we don't enable by default "Prevent accessibility services from accessing your browser" in the preferences: https://blog.torproject.org/comment/286871#comment-286871

Enabling this preference might make it more difficult for some 3rd party software to monitor what happens in the browser. Maybe we could enable this by default, while still allowing people who need accessibility service to change it.

It seems accessibility.force_disabled is the pref controlling this.