Fix AdsManager plugin not loading

Summary:
FluxStores plugin of AdsManager didn't open on Sandy. The underlying reason is that in Sandy the list was based on the plugin id's as exposed in the `package.json`, while in the old left sidebar it was based on the `id` stored on the Plugin class. And apparently they can differ (will fix that in the next diff)

We can release an update of this plugin after it landed

Reviewed By: passy

Differential Revision: D25302357

fbshipit-source-id: 874aab263fad6a83f63edc4a8f7fa7ceceaef70f

desktop/app/src/reducers/connections.tsx

@@ -519,6 +519,11 @@ function updateSelection(state: Readonly<State>): State {
   ) {
     // currently selected plugin is not available in this state,
     // fall back to the default
+    if (state.selectedPlugin) {
+      console.warn(
+        `Tried to open plugin '${state.selectedPlugin}' which is currently not available. Falling back to '${DEFAULT_PLUGIN}'`,
+      );
+    }
     updates.selectedPlugin = DEFAULT_PLUGIN;
   }