Improve typing of BackHandler (#48411)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/48411

## Changelog:
[Internal] -

Floow-up to https://github.com/facebook/react-native/pull/48388, based on the diff discussion (D67648077).

Adds a bit better typing to `BackHandler.js`.

Reviewed By: blakef

Differential Revision: D67713236

fbshipit-source-id: 95435898d8ea87f6ae32a6db859d6641e1264972

packages/react-native/Libraries/Utilities/BackHandler.android.js

@@ -14,12 +14,13 @@ import RCTDeviceEventEmitter from '../EventEmitter/RCTDeviceEventEmitter';
 const DEVICE_BACK_EVENT = 'hardwareBackPress';
 
 type BackPressEventName = 'backPress' | 'hardwareBackPress';
+type BackPressHandler = () => ?boolean;
 
-const _backPressSubscriptions = [];
+const _backPressSubscriptions: Array<BackPressHandler> = [];
 
 RCTDeviceEventEmitter.addListener(DEVICE_BACK_EVENT, function () {
   for (let i = _backPressSubscriptions.length - 1; i >= 0; i--) {
-    if (_backPressSubscriptions[i] && _backPressSubscriptions[i]()) {
+    if (_backPressSubscriptions[i]?.()) {
       return;
     }
   }
@@ -57,7 +58,7 @@ type TBackHandler = {|
   +exitApp: () => void,
   +addEventListener: (
     eventName: BackPressEventName,
-    handler: () => ?boolean,
+    handler: BackPressHandler,
   ) => {remove: () => void, ...},
 |};
 const BackHandler: TBackHandler = {
@@ -76,7 +77,7 @@ const BackHandler: TBackHandler = {
    */
   addEventListener: function (
     eventName: BackPressEventName,
-    handler: () => ?boolean,
+    handler: BackPressHandler,
   ): {remove: () => void, ...} {
     if (_backPressSubscriptions.indexOf(handler) === -1) {
       _backPressSubscriptions.push(handler);

packages/react-native/Libraries/Utilities/BackHandler.ios.js

@@ -4,13 +4,12 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
+ * @flow strict-local
  * @format
- * @flow
  */
 
-'use strict';
-
 type BackPressEventName = 'backPress' | 'hardwareBackPress';
+type BackPressHandler = () => ?boolean;
 
 function emptyFunction(): void {}
 
@@ -18,13 +17,13 @@ type TBackHandler = {|
   +exitApp: () => void,
   +addEventListener: (
     eventName: BackPressEventName,
-    handler: () => ?boolean,
+    handler: BackPressHandler,
   ) => {remove: () => void, ...},
 |};
 
 let BackHandler: TBackHandler = {
   exitApp: emptyFunction,
-  addEventListener(_eventName: BackPressEventName, _handler: Function) {
+  addEventListener(_eventName: BackPressEventName, _handler: BackPressHandler) {
     return {
       remove: emptyFunction,
     };