Fix Switch ref forwarding (facebook#31629)

Summary:
Since moving Switch to a function component it is no longer possible to get the native switch ref. This adds forwardRef so it is possible again.

## Changelog

[General] [Fixed] - Fix Switch ref forwarding

Pull Request resolved: facebook#31629

Test Plan: Tested the change in an app using react-native-gesture-handler, which tries to set a ref on Switch. Also made sure flow still passes.

Reviewed By: TheSavior

Differential Revision: D28839605

Pulled By: lunaleaps

fbshipit-source-id: 1fee86145caeabb60c0010bb9062dddca419f7ca

Libraries/Components/Switch/Switch.js

@@ -129,7 +129,13 @@ const returnsTrue = () => true;
   export default App;
   ```
  */
-export default function Switch(props: Props): React.Node {
+
+const SwitchWithForwardedRef: React.AbstractComponent<
+  Props,
+  React.ElementRef<
+    typeof SwitchNativeComponent | typeof AndroidSwitchNativeComponent,
+  >,
+> = React.forwardRef(function Switch(props, forwardedRef): React.Node {
   const {
     disabled,
     ios_backgroundColor,
@@ -144,9 +150,11 @@ export default function Switch(props: Props): React.Node {
   const trackColorForFalse = trackColor?.false;
   const trackColorForTrue = trackColor?.true;
 
-  const nativeSwitchRef = React.useRef<?React.ElementRef<
+  const nativeSwitchRef = React.useRef<React.ElementRef<
     typeof SwitchNativeComponent | typeof AndroidSwitchNativeComponent,
-  >>(null);
+  > | null>(null);
+  React.useImperativeHandle(forwardedRef, () => nativeSwitchRef.current);
+
   const [native, setNative] = React.useState({value: null});
 
   const handleChange = (event: SwitchChangeEvent) => {
@@ -228,4 +236,6 @@ export default function Switch(props: Props): React.Node {
       />
     );
   }
-}
+});
+
+export default SwitchWithForwardedRef;