Add Analytics setting to toggle reporting for "Demography and Interest for Remarketing and Advertising"

Site Kit should offer control about the Analytics configuration setting for whether reporting for "Demography and Interest for Remarketing and Advertising" is enabled or not.

Originally reported in #36

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## Acceptance criteria

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## Implementation Brief

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### Test Coverage

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### Visual Regression Changes

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## QA Brief

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## Changelog entry

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The issue names no files, tests, or entry points. Start by locating the Analytics configuration handling for the Demography and Interest reporting setting; done means users can toggle it and reporting follows that choice, with coverage for both enabled and disabled states.

Written by the indexing model from the issue text.