改编自oc版本的播放器
使用到的类主要有：
AVPlayerItem：
AVPlayer：播放器
AVPlayerLayer：预览页
核心方法：
let url = URL.init(string: "")
let playerItem =AVPlayerItem.init(url: url!)
let player =AVPlayer.init(playerItem: playerItem)
let playerLayer =AVPlayerLayer.init(player: player)
playerLayer.frame =view.frame
view.layer.addSublayer(playerLayer)
player.play()
切换视频：player.replaceCurrentItem(with: <#T##AVPlayerItem?#>)
停止播放：player.pause()
调整进度：player.seek(to: <#T##CMTime#>)
监听播放进度：
playerItem.addObserver(self, forKeyPath:"status", options: .new, context:nil)
监听缓存进度：
playerItem.addObserver(self, forKeyPath:"loadedTimeRanges", options: .new, context:nil)
监听播放状态：
NotificationCenter.default.addObserver(self, selector: #selector(self.playbackFinished(noti:)), name: NSNotification.Name.AVPlayerItemDidPlayToEndTime, object: player.currentItem)
