Separate the concepts of archived and disconnected devices

Summary:
Minor code cleanup to avoid future confusion:

- archived: a device that was imported from a Flipper trace, and only has persisted state
- (dis)connected: a real stateful device that might or might not have an active connection

Reviewed By: nikoant

Differential Revision: D26275459

fbshipit-source-id: eba554b37c39711e367c3795ff4456329a303c22

desktop/flipper-plugin/src/plugin/PluginBase.tsx

@@ -136,8 +136,7 @@ export abstract class BasePluginInstance {
         return realDevice.isArchived;
       },
       get isConnected() {
-        // for now same as isArchived, in the future we might distinguish between archived/imported and disconnected/offline devices
-        return !realDevice.isArchived;
+        return realDevice.connected.get();
       },
       deviceType: realDevice.deviceType,