Stabilize RootTagContext

Summary:
Changelog:
[General][Added] - Stabilize RootTagContext. And temporarily export both `unstable_RootTagContext` and `RootTagContext`

Reviewed By: TheSavior

Differential Revision: D27951427

fbshipit-source-id: dff8d4ca07c89edeeb517a42a3922e4e23899d8e

index.js

@@ -482,6 +482,9 @@ module.exports = {
   get unstable_RootTagContext(): RootTagContext {
     return require('./Libraries/ReactNative/RootTag').RootTagContext;
   },
+  get RootTagContext(): RootTagContext {
+    return require('./Libraries/ReactNative/RootTag').RootTagContext;
+  },
   get unstable_enableLogBox(): () => void {
     return () =>
       console.warn(

packages/rn-tester/js/examples/TurboModule/SampleTurboModuleExample.js

@@ -17,7 +17,7 @@ import {
   FlatList,
   Platform,
   TouchableOpacity,
-  unstable_RootTagContext,
+  RootTagContext,
 } from 'react-native';
 import * as React from 'react';
 
@@ -33,7 +33,7 @@ type State = {|
 |};
 
 class SampleTurboModuleExample extends React.Component<{||}, State> {
-  static contextType: React$Context<RootTag> = unstable_RootTagContext;
+  static contextType: React$Context<RootTag> = RootTagContext;
 
   state: State = {
     testResults: {},