[w3c/ServiceWorker] Split security and privacy considerations sections (#1562)

Current [best practice](https://w3ctag.github.io/security-questionnaire/#reviews) is to have separate security and privacy considerations sections. I'm not going to yell about the current structure, but if you're expanding the privacy analysis anyway, as I suggest in https://github.com/w3c/ServiceWorker/issues/1561, then perhaps split them entirely?

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