Change 'React' import writing, Prevent warnings in old version React (#4428)

src/navigators/createStackNavigator.js

@@ -1,4 +1,4 @@
-import * as React from 'react';
+import React from 'react';
 import createNavigationContainer from '../createNavigationContainer';
 import createKeyboardAwareNavigator from './createKeyboardAwareNavigator';
 import createNavigator from './createNavigator';

src/views/StackView/StackView.js

@@ -1,4 +1,4 @@
-import * as React from 'react';
+import React from 'react';
 import { NativeModules } from 'react-native';
 
 import StackViewLayout from './StackViewLayout';

src/views/StackView/StackViewLayout.js

@@ -1,4 +1,4 @@
-import * as React from 'react';
+import React from 'react';
 
 import clamp from 'clamp';
 import {

src/views/__tests__/Transitioner-test.js

@@ -1,5 +1,5 @@
 /* eslint react/display-name:0 */
-import * as React from 'react';
+import React from 'react';
 import renderer from 'react-test-renderer';
 import Transitioner from '../Transitioner';