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)