The first time startdiscovery is called, some discovery results are lost

### Problem summary
When i use `FlutterBluetoothSerial.instance.startDiscovery().listen((bluetoothDiscoveryResult) {//...});` for the first time, I see that the plugin logs some discovered devices, which are not transmitted to the stream i am listening below. If I restart the discovery it is fine then.

### Steps to reproduce
To reproduce this issue, simply call `FlutterBluetoothSerial.instance.startDiscovery().listen((bluetoothDiscoveryResult) {//...});` and print the results device mac address. Then compare what you see with the plugin logs.
### Environment
I use
-Flutter Channel stable, 2.5.3
-Android SDK version 31.0.0
### Potential cause
I tried swapping this two blocks of code in FlutterBluetoothSerial from the library:
```
await _methodChannel.invokeMethod('startDiscovery');

subscription = _discoveryChannel.receiveBroadcastStream().listen(
controller.add,
onError: controller.addError,
onDone: controller.close,
);
```
like so:
```
subscription = _discoveryChannel.receiveBroadcastStream().listen(
controller.add,
onError: controller.addError,
onDone: controller.close,
);

await _methodChannel.invokeMethod('startDiscovery');
```

It seems to work for my problem but I am unsure if this is correct/safe. I tried this change because receiveBroadcastStream() returns a broacast stream which will loose its data if no one listens to it!

Start in the FlutterBluetoothSerial startDiscovery implementation, focusing on the ordering of receiveBroadcastStream().listen and invokeMethod('startDiscovery'). Reproduce the first-discovery case from the issue and compare stream results with plugin logs; done when the initial discovery delivers all logged devices without breaking later discoveries.

Written by the indexing model from the issue text.