Fixing the missing devices when discovering for the first time. Fixes #151

[ncdm-stldr]

The first time one call startDiscovery(), some results are lost and not sent through the stream returned by startDiscovery.
After investigation, it appeared to me this happens because the plugin calls _methodChannel.invokeMethod('startDiscovery'); before listening to the EventChannel _discoveryChannel.
Therefore I switched the two code blocks. I also added something to prevent the following situation:

• We listen to the _discoveryChannel EventChannel.
• An past discovery finishes now which triggers the onDone method of the subsciption we just created
• We start a discovery.
• We return a closed empty stream which will never receive the result of the new discovery

[ncdm-stldr]

Note that to reproduce the issue, one can simply run the example application of the plugin, then explore the scanned devices: the first time we click on the button, we do not see all the devices. We can however see some devices in the logs which are not displayed because startDiscovery doesn't return them.

[ncdm-stldr]

@DanielLimaFN
Hi,
Thank you the review.
Is this repository still maintained?
It seems that there are a lots of people that would be interested to contribute to the project. For example there are at least 5 forks (including yours) that support android 12.
Is there already another repository that replaced the one from edufolly?