Add spec for KeyboardObserver (#24881)

Summary:
part of #24875. iOS only it appears, but not really used by RN itself. Should be fine?

## Changelog

[General] [Added] - Add TM spec for KeyboardObserver
Pull Request resolved: https://github.com/facebook/react-native/pull/24881

Reviewed By: fkgozali

Differential Revision: D15391769

Pulled By: rickhanlonii

fbshipit-source-id: 557507f6063b40d1c68ec8739e23b33bc22ade39

Libraries/Components/Keyboard/Keyboard.js

@@ -13,10 +13,10 @@
 const LayoutAnimation = require('../../LayoutAnimation/LayoutAnimation');
 const invariant = require('invariant');
 const NativeEventEmitter = require('../../EventEmitter/NativeEventEmitter');
-const KeyboardObserver = require('../../BatchedBridge/NativeModules')
-  .KeyboardObserver;
 const dismissKeyboard = require('../../Utilities/dismissKeyboard');
-const KeyboardEventEmitter = new NativeEventEmitter(KeyboardObserver);
+
+import NativeKeyboardObserver from './NativeKeyboardObserver';
+const KeyboardEventEmitter = new NativeEventEmitter(NativeKeyboardObserver);
 
 export type KeyboardEventName =
   | 'keyboardWillShow'

Libraries/Components/Keyboard/NativeKeyboardObserver.js

@@ -0,0 +1,21 @@
+/**
+ * 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
+ * @format
+ */
+
+'use strict';
+
+import type {TurboModule} from 'RCTExport';
+import * as TurboModuleRegistry from 'TurboModuleRegistry';
+
+export interface Spec extends TurboModule {
+  +addListener: (eventName: string) => void;
+  +removeListeners: (count: number) => void;
+}
+
+export default TurboModuleRegistry.get<Spec>('KeyboardObserver');