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/reducers/settings.tsx

@@ -9,9 +9,21 @@
 
 import {Actions} from './index';
 
+export enum Tristate {
+  True,
+  False,
+  Unset,
+}
+
 export type Settings = {
   androidHome: string;
   enableAndroid: boolean;
+  /**
+   * If unset, this will assume the value of the GK setting.
+   * Note that this setting has no effect in the open source version
+   * of Flipper.
+   */
+  enablePrefetching: Tristate;
 };
 
 export type Action =
@@ -24,6 +36,7 @@ export type Action =
 const initialState: Settings = {
   androidHome: '/opt/android_sdk',
   enableAndroid: true,
+  enablePrefetching: Tristate.Unset,
 };
 
 export default function reducer(