Make sure Sandy plugins are selectable during export

Summary:
This diff makes sure Sandy plugins show up as well in the plugin selector when making exports (and in support form as well).

Also verified that this works with the Sandy updated section plugin.

Note that persisted state now didn't need any changes in the plugin code to work :)

Commented / simplified the calculation of available plugins a little bit and fixed a confusing issue where two different redux stores where created in one unit test, which caused an issue in the new implementation.

Reviewed By: jknoxville

Differential Revision: D22434301

fbshipit-source-id: c911196bc5b105309e82204188f124f40aab487a

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

@@ -217,6 +217,10 @@ export class SandyPluginInstance {
     );
   }
 
+  isPersistable(): boolean {
+    return Object.keys(this.rootStates).length > 0;
+  }
+
   private assertNotDestroyed() {
     if (this.destroyed) {
       throw new Error('Plugin has been destroyed already');