Summary:
Introduced `isConnected` flag on device and plugin client to reflect whether a connection is still available for the plugins, or that they have been disconnected.
Potentially we could expose the (readonly) `connected` state atom for this as well, or an `onDisconnect` event for device pugins, to create a responsive UI, but there might be no need for that, in which case this suffices.
Reviewed By: nikoant
Differential Revision: D26249346
fbshipit-source-id: b8486713fdf2fcd520488ce54f771bd038fd13f8
bb529411b5