This replaces `UrlbarResult.exposureResultType` and `exposureResultHidden` with
a single simple value called `exposureTelemetry`. Please see the bug for the
rationale.
Other changes:
* For convenience, make `UrlbarPrefs.get("exposureResults")` return a `Set` so
consumers don't have to parse the string value.
* Simplify handling of result properties in `check_results()` (xpcshell tests)
* Add more tasks/checks to test_exposure.js
Depends on D219939
Differential Revision: https://phabricator.services.mozilla.com/D219942
26cdd4f115