Addresses an issue with Flipper message identifier

Summary:
This change addresses a problem with using data.id straight into the if statement.

If `data.id` is not undefined but has a value of 0 then the expression evaluates to false.

Right now, that wasn't the intended usage.

Using this change to also export some types from Client.

Reviewed By: passy

Differential Revision: D30069746

fbshipit-source-id: 04a8d161efceadf7a38ee556be70c15b45c11056

desktop/app/src/Client.tsx

@@ -54,12 +54,16 @@ export type ClientExport = {
   query: ClientQuery;
 };
 
-type Params = {
+export type Params = {
   api: string;
   method: string;
   params?: Object;
 };
-type RequestMetadata = {method: string; id: number; params: Params | undefined};
+export type RequestMetadata = {
+  method: string;
+  id: number;
+  params: Params | undefined;
+};
 
 const handleError = (store: Store, device: BaseDevice, error: ErrorType) => {
   if (store.getState().settingsState.suppressPluginErrors) {

desktop/app/src/comms/ServerWebSocket.tsx

@@ -133,7 +133,7 @@ class ServerWebSocket extends ServerWebSocketBase {
         error?: ErrorType | undefined;
       } = json;
 
-      if (data.hasOwnProperty('id') && data.id) {
+      if (data.hasOwnProperty('id') && data.id !== undefined) {
         const callbacks = pendingRequests.get(data.id);
         if (!callbacks) {
           return;