Bug 1436226: Ignore user prefs and hardware support for media capabilities...

Bug 1436226: Ignore user prefs and hardware support for media capabilities when RFPTarget::MediaCapabilities is enabled. r=tjr,media-playback-reviewers,padenot

This patch mostly targeted Android, as media.mediasource.vp9.enabled is disabled on only Android and HW support takes precedence over pref, hence leaking HW support for VP9. However, we ended up modifying the patch to ignore prefs or HW support, fixing both possible user pref leak and HW support leak.

Differential Revision: https://phabricator.services.mozilla.com/D221338