Bug 2003370 - Use and react to Gecko preferences stored values

[ohall-m]

This patch adds functionality to use the new PreviousGeckoPrefState.

It adds:

• set_gecko_prefs_original_values for external handling of setting prefs back to original values
• Mechanisms to return to a previous states when:
  • on_experiment_updated
    • Certain situations and as determined in will_pref_experiment_change
  • on_experiment_ended
  • on_opt_out

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[ohall-m]

Hey @freshstrangemusic and @jeddai, just leaving a message that part 2 to use the stored values is up because I can't set reviewers!

The observer part 3 will be in bug 2006095.

[ohall-m]

I didn't add anything in unenroll_for_gecko_pref because this is what the observer will call into in the event of an external gecko pref change and we do not want to revert the pref in that case. Mentioning it as a heads up because I think it might be a bit unexpected from certain standpoints - like it almost looks like a utility that handles all the pref stuff in one swoop. I keep revisiting it and am afraid it could be confusing down the road.