Bug 1838208 - Part 1: Add pref to allow choosing exception strategy....

Bug 1838208 - Part 1: Add pref to allow choosing exception strategy. r=sefeng,necko-reviewers, a=dmeehan

In `IsOpaqueSafeListedSpecBreakingMIMEType` we make it possible to
configure, with pref, what kind of exception strategy to use to handle
compatibility problems with MIME types starting with `audio/` or
`video/`.

Differential Revision: https://phabricator.services.mozilla.com/D180926