Fix ID used when listening to updates on new sites (#193)

This should avoid the errors we've been seeing in Sentry of:

```
MissingPluginException(No implementation found for method listen on channel net.defined.nebula/null)
```

To test, start up the mobile app in an android phone or emulator, tap the plus button to add a new site, and verify that no error log is shown.

Note: we still get a similar error when tapping the "Good Site" or "Bad Site" in debug, but I believe that is because it's being created too quickly before the EventChannel has a chance to be set up.  Adding a site manually does not trigger the error.

android/app/src/main/kotlin/net/defined/mobile_nebula/MainActivity.kt

@@ -54,12 +54,11 @@ class MainActivity: FlutterActivity() {
     }
 
     override fun configureFlutterEngine(flutterEngine: FlutterEngine) {
+        super.configureFlutterEngine(flutterEngine)
         appContext = context
         //TODO: Initializing in the constructor leads to a context lacking info we need, figure out the right way to do this
         sites = Sites(flutterEngine)
-
-        GeneratedPluginRegistrant.registerWith(flutterEngine)
-
+        
         ui = MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL)
         ui!!.setMethodCallHandler { call, result ->
             when(call.method) {
@@ -99,7 +98,7 @@ class MainActivity: FlutterActivity() {
 
         apiClient = APIClient(context)
 
-        ContextCompat.registerReceiver(context, refreshReceiver, IntentFilter(ACTION_REFRESH_SITES), RECEIVER_NOT_EXPORTED)
+        ContextCompat.registerReceiver(context, refreshReceiver, IntentFilter(ACTION_REFRESH_SITES), ContextCompat.RECEIVER_NOT_EXPORTED)
 
         enqueueDNUpdater()
     }

lib/models/Site.dart

@@ -93,7 +93,7 @@ class Site {
     this.rawConfig = rawConfig;
     this.lastManagedUpdate = lastManagedUpdate;
 
-    _updates = EventChannel('net.defined.nebula/$id');
+    _updates = EventChannel('net.defined.nebula/${this.id}');
     _updates.receiveBroadcastStream().listen((d) {
       try {
         _updateFromJson(d);