Replacing all instances of [UIApplication sharedApplication] wi…

Summary:
…th RCTSharedApplication()

Thanks for submitting a PR! Please read these instructions carefully:

- [ ] Explain the **motivation** for making this change.
          Using React Native latest version with Cocoapods 1.2.0 causes the following error inside iOS app extensions

> /react-native/React/Modules/RCTAccessibilityManager.m:67:70: ‘sharedApplication’ is unavailable: not available on iOS (App Extension) — Use view controller based solutions where appropriate instead.

Moving the use of [UIApplication sharedApplication] to RCTSharedApplication() which is safe on app extension

- [ ] Provide a **test plan** demonstrating that the code is solid.
        I am not sure how to test such that all the features which touch the modified code are tested.

- [ ] Match the **code formatting** of the rest of the codebase.
- [ ] Target the `master` branch, NOT a "stable" branch.

What existing problem does the pull request solve?
    Using React Native latest v
Closes https://github.com/facebook/react-native/pull/13227

Differential Revision: D4816338

Pulled By: javache

fbshipit-source-id: e3e3c77882990ad1817b0b633521cff52571ecd0

React/Modules/RCTStatusBarManager.m

@@ -99,7 +99,7 @@ RCT_EXPORT_MODULE()
 RCT_EXPORT_METHOD(getHeight:(RCTResponseSenderBlock)callback)
 {
   callback(@[@{
-    @"height": @([UIApplication sharedApplication].statusBarFrame.size.height),
+    @"height": @(RCTSharedApplication().statusBarFrame.size.height),
   }]);
 }