Add launcher prefetch setting

Summary: Allows users to override the GK status of prefetching. It is implemented as a Tri-State. "Unset" means that the local Flipper config won't take precedent over the GK setting and will leave it unchanged. If a user interacts with the setting, it gets persisted into the config and will from then-on override the GK, meaning that the user has an opt-in mechanism and if we open the GK to more people, they effectively have an opt-out.

Reviewed By: jknoxville

Differential Revision: D18008259

fbshipit-source-id: bdfde9a8b9acf43aa60c84800a7979a29a4e9364

src/fb-stubs/LauncherSettingsPanel.tsx

@@ -0,0 +1,17 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ */
+
+import {Tristate} from 'src/reducers/settings';
+
+export default function(_props: {
+  enabledInConfig: Tristate;
+  onChange: (v: Tristate) => void;
+}) {
+  return null;
+}

src/fb-stubs/Prefetcher.tsx

@@ -8,3 +8,4 @@
  */
 
 export default async function setupPrefetcher() {}
+export const shouldInstallPrefetcher = () => false;