android scroll style

Like android's scroll bars. Forked from onlywsx's extension. Current issue: due to unsupported API of native hiding scrollbar in firefox, the method of replacing scroll-bar from this add-on will cause incompatible with autopage-like add-ons. And some websites will have slight uncorrect layouts at margin. Maybe try to fix later. Sorry for some bugs currently. 像android的滚动条。移植自onlywsx的原chrome扩展。 目前已知问题，由于firefox原生的隐藏滚动条api启用后不支持滚动，所以目前此附加组件使用的方法将会造成与自动翻页类的组件如autopagerize的不兼容，以及个别网站的右边底部边缘可能存在些许排版错误。以后可能修复。很抱歉目前存在一些bug。