[ios][tools] Do not add duplicate bonjour services to Info.plist

### Steps to reproduce

I was just reading the code, and there is a logic error here:

https://github.com/flutter/flutter/blob/5b78ecca51f0f37ce2ac2deec77cb8473b8a9e58/packages/flutter_tools/bin/xcode_backend.dart#L419-L435

If the bonjour services is already setup properly, we shouldn't add duplicate `_dartVmService._tcp` entry to it.

### Expected results

No duplicate entries in bonjour services

### Actual results

I wrote a simple test and got duplicate values:

```
'\tNSBonjourServices\n'
'\t\n'
'\t\t_dartVmService._tcp\n'
'\t\t_dartVmService._tcp\n'
'\t\t_bogus._tcp\n'
'\t\n'
```

### Code sample

NA

### Screenshots or Video

NA

### Logs

NA

### Flutter Doctor output

NA