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.h

@@ -74,6 +74,8 @@ class FlipperClient : public FlipperConnectionManager::Callbacks {
 
   void onDisconnected() override;
 
+  bool isConnected();
+
   void onMessageReceived(
       const folly::dynamic& message,
       std::unique_ptr<FlipperResponder>) override;