Add spec for NativeDeviceEventManager (#24914)

Summary:
Part of #24875.

## Changelog

[General] [Added] - TM add spec for NativeDeviceEventManager
Pull Request resolved: https://github.com/facebook/react-native/pull/24914

Reviewed By: RSNara

Differential Revision: D15421462

Pulled By: fkgozali

fbshipit-source-id: 78b90c615cd031ecd010a552751e506406ab739a

Libraries/NativeModules/specs/NativeDeviceEventManager.js

@@ -0,0 +1,20 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * 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
+ */
+
+'use strict';
+
+import type {TurboModule} from 'RCTExport';
+import * as TurboModuleRegistry from 'TurboModuleRegistry';
+
+export interface Spec extends TurboModule {
+  +invokeDefaultBackPressHandler: () => void;
+}
+
+export default TurboModuleRegistry.get<Spec>('DeviceEventManager');

Libraries/Utilities/BackHandler.android.js

@@ -10,9 +10,8 @@
 
 'use strict';
 
-const DeviceEventManager = require('../BatchedBridge/NativeModules')
-  .DeviceEventManager;
-const RCTDeviceEventEmitter = require('../EventEmitter/RCTDeviceEventEmitter');
+import NativeDeviceEventManager from '../../Libraries/NativeModules/specs/NativeDeviceEventManager';
+import RCTDeviceEventEmitter from '../EventEmitter/RCTDeviceEventEmitter';
 
 const DEVICE_BACK_EVENT = 'hardwareBackPress';
 
@@ -73,7 +72,11 @@ type TBackHandler = {|
 |};
 const BackHandler: TBackHandler = {
   exitApp: function(): void {
-    DeviceEventManager.invokeDefaultBackPressHandler();
+    if (!NativeDeviceEventManager) {
+      return;
+    }
+
+    NativeDeviceEventManager.invokeDefaultBackPressHandler();
   },
 
   /**