Replace pref checking with embedded experiment API

[jaredhirsch]

The history.enabled pref can be checked via the privacy API (#4531), but other prefs should be checked via an embedded webextension API.

Related to #3875.

[chenba]

I asked in #webextensions if WebExtension experiments would work if they are in an embedded extension, and kmag said "probably not". And that was my experience. The experiment API (browser.whateverExpierment) was undefined when I tried using it.

[ianb]

The add-on can run as a WebExtension instead of a bootstrap extension, I guess testable with ./bin/run-addon --webextension

That means we can test, though we can't incrementally deploy web experiment APIs until we're ready to turn off bootstrap entirely.

[jaredhirsch]

Closing in favor of #4773