lock dom.user_activation.transient.timeout pref to 5000 (ms)

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1791079

Adds UserActivation API. A web page could probably get similar information before, indirectly. But maybe we should check that the API doesn't have any preference-specific behaviour underneath. E.g. the amount of time it takes for UserActivation.isActive to switch from true to false.