New 'isConnected' API

Summary:
Expose a new API to be used to check if there's an open connection with Flipper Desktop.

Changelog: new FlipperClient isConnected API

Reviewed By: antonk52

Differential Revision: D46841095

fbshipit-source-id: 82a60f52496fb218cb50c6a28d7ffe7225ae23aa

xplat/Flipper/FlipperClient.cpp

@@ -160,6 +160,13 @@ void FlipperClient::onDisconnected() {
   });
 }
 
+bool FlipperClient::isConnected() {
+  if (socket_ != nullptr) {
+    return socket_->isConnected();
+  }
+  return false;
+}
+
 void FlipperClient::onMessageReceived(
     const dynamic& message,
     std::unique_ptr<FlipperResponder> uniqueResponder) {