Review the SOCKS preference update

In one of the refactors that lead to TorProvider, a few important points about settings the SOCKS preferences emerged (see !721 (comment 2934453)):

• it isn't clear why we first read, do stuff and then write
  • might be needed if we want not to hardcode the SOCKS port number, see #40930
• environment variables will change the value of a pref, so they're not consistent with the control port env variables, that are one-time only
  • we could/should use another set of preferences that don't overlap with the browser's settings