how to use resource cache with flutter_webview_plugin

I verify `appCacheEnabled: true` ,
and ``.

but still I can't use cache.

**behavior**:
when app is running, load image only once.
but when I close the app, running again. it will download image or other resource, i check cache storage with application information, it not change storage size.
so, I think cache is only used once.

how can I do?

首先检查 flutter_webview_plugin 对 appCacheEnabled 和 Android INTERNET 权限的处理，然后在应用重启后复现所报告的行为。未指定源文件或测试；完成标准是确定是否支持持久的资源缓存，并记录或修正观察到的行为。

由索引模型根据 Issue 内容生成。