Backport Bugzilla 1436226 Hardcode VP8/VP9

https://bugzilla.mozilla.org/show_bug.cgi?id=1436226

we should also remove/revisit setting prefs like media.benchmark.vp9.threshold now that capabilities are hardcoded

also note emphasis mine 😄

It may call IsV9Forced but that requires user disabling media.mediasource.vp9.enabled, which the Tor browser can lock to true on Desktop, but on Android things are different.

cc @pierov