Expose current connection status to Sandy plugins

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

desktop/flipper-plugin/src/test-utils/test-utils.tsx

@@ -38,6 +38,7 @@ import {BasePluginInstance} from '../plugin/PluginBase';
 import {FlipperLib} from '../plugin/FlipperLib';
 import {stubLogger} from '../utils/Logger';
 import {Idler} from '../utils/Idler';
+import {createState} from '../state/atom';
 
 type Renderer = RenderResult<typeof queries>;
 
@@ -207,10 +208,13 @@ export function startPlugin<Module extends FlipperPluginModule<any>>(
     isBackgroundPlugin(_pluginId: string) {
       return !!options?.isBackgroundPlugin;
     },
+    connected: createState(false),
     initPlugin() {
+      this.connected.set(true);
       pluginInstance.connect();
     },
     deinitPlugin() {
+      this.connected.set(false);
       pluginInstance.disconnect();
     },
     call(