Bug 1797580 - Fix broken assumptions with allowlists and the downloadable blocklist. r=jrmuizel

Ensure that when we evaluate the downloadable blocklist, we actually
only use the downloadable blocklist. We should not include any platform
specific checks in these prefs, as it causes confusion about why the
prefs were set in the first place. Allowlisted features should be
ignored when evaluating the downloadable blocklist; if we wish to
override the ALLOW/ALLOW_QUALIFIED/DENIED statuses, we should use OK or
BLOCKED_DEVICE or similar instead.

This caused allowlisted features (like WebRender) to be taken away from
users in the most recent nightly.

Differential Revision: https://phabricator.services.mozilla.com/D160408