禁止自动播放音频视频有时候失效

This pref defines what the blocking policy would be used in blocking autoplay.
0 : use sticky activation (default) https://html.spec.whatwg.org/multipage/interaction.html#sticky-activation
1 : use transient activation (the transient activation duration can be adjusted by the pref `dom.user_activation.transient.timeout`) https://html.spec.whatwg.org/multipage/interaction.html#transient-activation
2 : user input depth (allow autoplay when the play is trigged by user input which is determined by the user input depth)