Microphone is not working in background (IOS)

## System info

Issue occurs on: iOS (14+)
Plugin version: 0.4.0
Flutter doctor output:

```
here is ok
```

## Steps to Reproduce

1. Join any webRTC meeting via webview (ex: https://whereby.com/)
2. Tap on Home button on your IOS device
3. Now you are able to listen only (microphone is not work in background). If you reproduce this actions with Safari, you get green call overlay on top (and audio and micro would continue work is background)

首先，按照列出的 WebView 会议步骤，在 iOS 14+ 上使用插件版本 0.4.0 重现该问题，并将其行为与 Safari 进行比较。当按下 Home 按钮后麦克风音频仍能继续，同时保持现有的监听行为和绿色通话叠加层预期时，工作即完成。

由索引模型根据 Issue 内容生成。