Normalize the media preload behavior

See https://bugzilla.mozilla.org/show_bug.cgi?id=1969210#c6.

There are a few preferences we might want to normalize for defense-in-depth: media.preload.default, media.preload.default.cellular, media.preload.auto, and media.preload.auto.cellular.

It seems they affect mostly Android but not us, since we don't get information about the network state. However, if we do it for getting the Internet status (see #41661), we might want to add protections also at a lower layer.