Fixes subscription to _deviceConnector.deviceConnectionStateUpdateStream leaking (#876)

- Previous code would create a broadcastStream from a normal stream (created by Repeater).
When this is done, the source stream subscription needs to be explicitly closed.
This is usually done by implementing the onCancel callback from the asBroadcastStream method.
This was not happening, so every call to connectedDeviceStream was creating a new subscription that was never closed.
- To fix the problem, this commit uses the Repeater.broadcast constructor which already returns a broadcast stream
and also closes the source stream subscription when needed

Author: gabrielgarciagava

packages/flutter_reactive_ble/lib/src/reactive_ble.dart

@@ -60,11 +60,10 @@ class FlutterReactiveBle {
   BleStatus get status => _status;
 
   /// A stream providing connection updates for all the connected BLE devices.
-  Stream<ConnectionStateUpdate> get connectedDeviceStream => Repeater(onListenEmitFrom: () async* {
+  Stream<ConnectionStateUpdate> get connectedDeviceStream => Repeater.broadcast(onListenEmitFrom: () async* {
         await initialize();
         yield* _deviceConnector.deviceConnectionStateUpdateStream;
-      }).stream.asBroadcastStream()
-        ..listen((_) {});
+      }).stream;
 
   /// A stream providing value updates for all the connected BLE devices.
   ///
@@ -105,8 +104,7 @@ class FlutterReactiveBle {
       );
 
       if (Platform.isAndroid || Platform.isIOS) {
-        ReactiveBlePlatform.instance =
-            const ReactiveBleMobilePlatformFactory().create(
+        ReactiveBlePlatform.instance = const ReactiveBleMobilePlatformFactory().create(
           logger: _debugLogger,
         );
       }
@@ -398,11 +396,10 @@ class FlutterReactiveBle {
   Future<void> clearGattCache(String deviceId) =>
       _blePlatform.clearGattCache(deviceId).then((info) => info.dematerialize());
 
-  /// Reads the RSSI of the of the peripheral with the given device ID. 
+  /// Reads the RSSI of the of the peripheral with the given device ID.
   /// The peripheral must be connected, otherwise a [PlatformException] will be
   /// thrown
-  Future<int> readRssi(String deviceId) async =>
-      _blePlatform.readRssi(deviceId);
+  Future<int> readRssi(String deviceId) async => _blePlatform.readRssi(deviceId);
 
   /// Subscribes to updates from the characteristic specified.
   ///