fix comments bug when changing episode

[ErBWs]

不保存状态了，用 Sliver 的懒加载解决卡顿问题

另外现在的播放器有两个问题

• 手机端切换全屏会导致 player_item_panel 消失

• 控制台报异常(这个把 player_item_panel 的 build 里最顶层的 Expanded 删了可以解决)

======== Exception caught by widgets library =======================================================
The following assertion was thrown while applying parent data.:
Incorrect use of ParentDataWidget.

The ParentDataWidget Expanded(flex: 1) wants to apply ParentData of type FlexParentData to a RenderObject, which has been set up to accept ParentData of incompatible type StackParentData.

Usually, this means that the Expanded widget has the wrong ancestor RenderObjectWidget. Typically, Expanded widgets are placed directly inside Flex widgets.
The offending Expanded is currently placed inside a Stack widget.

The ownership chain for the RenderObject that received the incompatible parent data was:
  Stack ← Observer ← Expanded ← PlayerItemPanel ← Stack ← SizedBox ← Listener ← MouseRegion ← ColoredBox ← Container ← ⋯

[Predidit]

那我们可以把 Expanded 删掉，看上去不会有什么问题

[ErBWs]

但是第一个问题不知道什么原因，控制台也没报错，所以我就没改这个文件

[Predidit]

是我的问题，我考虑一下怎么修