Add ExPlat (A/B testing) support to Tracks

[leandroalonso]

The goal is to add support to the ExPlat (A/B testing platform)

Tasks

• Request wpcom/v2/experiments/0.1.0/assignments/<platform> and store the experiments values (Add initial A/B testing support WordPress-iOS#15211)
• Request the above again when the ttl has been reached (take into account closing/reopening the app)
• Add anonymous support
• TBD: Privacy

[leandroalonso]

@aaronyan @astralbodies question regarding Privacy: some users opt-out to all the tracking we do. In this case, I'd personally not even perform this request. Does that sound reasonable for you two?

[aaronyan]

I think that sounds reasonable to not request an experiment assignment for users that opt-out. I don't think this introduces any bias into the experiment, but I'll think about it. Is the no request strategy how Tracks events are handled for users that opt-out?

[leandroalonso]

@aaronyan I just checked here and, yes, if the user opt-out we don't perform the Tracks request. (@astralbodies please correct me if I'm wrong).