Support receiving messages in Sandy plugins

Summary: This diffs adds the capability to listen to messages in Sandy plugins. Although API wise it looks more like the old `this.subscribe`, semantically it behaves like the `persistedStateReducer`; messages are queued if the plugin is enabled but not active.

Reviewed By: nikoant

Differential Revision: D22282711

fbshipit-source-id: 885faa702fe779ac8d593c1d224b2be13e688d47

desktop/app/src/utils/pluginStateRecorder.tsx

@@ -32,6 +32,10 @@ function initialRecordingState(): typeof pluginRecordingState {
   };
 }
 
+export function isRecordingEvents(pluginKey: string) {
+  return pluginRecordingState.recording === pluginKey;
+}
+
 export function flipperRecorderAddEvent(
   pluginKey: string,
   method: string,