Fix FlutterBlue.state stream cancel

Fix bug where if there were multiple subscribers to FlutterBlue.state
and one cancelled it would accidentally cancel all subscribers
Fix for: pauldemarco#608

lib/src/flutter_blue.dart

@@ -12,6 +12,13 @@ class FlutterBlue {
   Stream<MethodCall> get _methodStream => _methodStreamController
       .stream; // Used internally to dispatch methods from platform.
 
+  /// Fix for issue #608
+  /// Cached broadcast stream for FlutterBlue.state events
+  /// Caching this stream allows for more than one listener to subscribe
+  /// and unsubscribe apart from each other,
+  /// while allowing events to still be sent to others that are subscribed
+  Stream<BluetoothState> _stateStream;
+
   /// Singleton boilerplate
   FlutterBlue._() {
     _channel.setMethodCallHandler((MethodCall call) {
@@ -51,10 +58,12 @@ class FlutterBlue {
         .then((buffer) => new protos.BluetoothState.fromBuffer(buffer))
         .then((s) => BluetoothState.values[s.state.value]);
 
-    yield* _stateChannel
+    _stateStream ??= _stateChannel
         .receiveBroadcastStream()
         .map((buffer) => new protos.BluetoothState.fromBuffer(buffer))
-        .map((s) => BluetoothState.values[s.state.value]);
+        .map((s) => BluetoothState.values[s.state.value]).doOnCancel(() => _stateStream = null);
+
+    yield* _stateStream;
   }
 
   /// Retrieve a list of connected devices