Bug 1997373 - Store original pref values for Fenix Gecko integration

[ohall-m]

This patch begins storing a PreviousState on ExperimentEnrollment when it is of type EnrollmentStatus::Enrolled. It also introduces PreviousState::GeckoPref to hold Gecko original preference values for Gecko pref based experiments.

The public APIs it opens are registerPreviousGeckoPrefStates and getPreviousState.

The goal of this patch is to appropriately store the original Gecko preference value to help when exiting an experiment.

Current idea is that it will be set similarly as unenrollForGeckoPref in this patch D257390, but for the positive case. Bug 2003370 will use this to properly consume these values when appropriate.

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[ohall-m]

Hi @jeddai and @freshstrangemusic ,

Please take a look at this when you have a chance! I don't have the right repo access to request in the Reviewers panel.

[ohall-m]

First draft comments should be addressed now, thanks!