Add Opt-in Switch to Sandy UI

Summary:
With the new design from vpileggi, this diff separated a new app UI to the current one.

This diffs show the toggle switch to enable Sandy UI for devs who are in `flipper_sandy` GK. When toggled, it will bring Sandy UI up.

Reviewed By: mweststrate

Differential Revision: D23599398

fbshipit-source-id: d85c707e0fe7726a418b3551cedb36e455fb7d14

desktop/app/src/chrome/SettingsSheet.tsx

@@ -25,6 +25,7 @@ import KeyboardShortcutInput from './settings/KeyboardShortcutInput';
 import {isEqual} from 'lodash';
 import restartFlipper from '../utils/restartFlipper';
 import LauncherSettingsPanel from '../fb-stubs/LauncherSettingsPanel';
+import SandySettingsPanel from '../fb-stubs/SandySettingsPanel';
 import {reportUsage} from '../utils/metrics';
 
 const Container = styled(FlexColumn)({
@@ -183,6 +184,17 @@ class SettingsSheet extends Component<Props, State> {
             });
           }}
         />
+        <SandySettingsPanel
+          toggled={this.state.updatedSettings.enableSandy}
+          onChange={(v) => {
+            this.setState({
+              updatedSettings: {
+                ...this.state.updatedSettings,
+                enableSandy: v,
+              },
+            });
+          }}
+        />
         <ToggledSection
           label="React Native keyboard shortcuts"
           toggled={reactNative.shortcuts.enabled}