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 facebook/react-native#13227

Differential Revision: D4816338

Pulled By: javache

fbshipit-source-id: e3e3c77882990ad1817b0b633521cff52571ecd0

templates/HelloWorld/ios/HelloWorldTests/HelloWorldTests.m

@@ -37,7 +37,7 @@ - (BOOL)findSubviewInView:(UIView *)view matching:(BOOL(^)(UIView *view))test
 
 - (void)testRendersWelcomeScreen
 {
-  UIViewController *vc = [[[[UIApplication sharedApplication] delegate] window] rootViewController];
+  UIViewController *vc = [[[RCTSharedApplication() delegate] window] rootViewController];
   NSDate *date = [NSDate dateWithTimeIntervalSinceNow:TIMEOUT_SECONDS];
   BOOL foundElement = NO;