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/chrome/SettingsSheet.tsx

@@ -18,6 +18,7 @@ import ToggledSection from './settings/ToggledSection';
 import {FilePathConfigField, ConfigText} from './settings/configFields';
 import isEqual from 'lodash.isequal';
 import restartFlipper from '../utils/restartFlipper';
+import LauncherSettingsPanel from '../fb-stubs/LauncherSettingsPanel';
 
 const Container = styled(FlexColumn)({
   padding: 20,
@@ -102,6 +103,17 @@ class SettingsSheet extends Component<Props, State> {
             frozen
           />
         </ToggledSection>
+        <LauncherSettingsPanel
+          enabledInConfig={this.state.updatedSettings.enablePrefetching}
+          onChange={v => {
+            this.setState({
+              updatedSettings: {
+                ...this.state.updatedSettings,
+                enablePrefetching: v,
+              },
+            });
+          }}
+        />
         <br />
         <FlexRow>
           <Spacer />