-[RCTAccessibilityManager announceForAccessibility:] calls into AppKit from background thread (#1852)

Specifically, NSAccessibilityPostNotificationWithUserInfo. Other methods such as `getCurrentHighContrastState` provide cached state (cached on main thread via `observeValueForKeyPath` et al). Some other methods such as `setAccessibilityFocus` dispatch to the main thread.

Not doing so could lead to AppKit further firing selectors on UI logic in app code and lead to downstream issues.

Author: nakambo

React/Modules/MacOS/RCTAccessibilityManager.m

@@ -60,13 +60,15 @@ - (void)dealloc
 
 RCT_EXPORT_METHOD(announceForAccessibility:(NSString *)announcement)
 {
+  dispatch_async(dispatch_get_main_queue(), ^{
     NSAccessibilityPostNotificationWithUserInfo(
                                                     NSApp,
                                                     NSAccessibilityAnnouncementRequestedNotification,
                                                     @{NSAccessibilityAnnouncementKey : announcement,
                                                       NSAccessibilityPriorityKey : @(NSAccessibilityPriorityHigh)
                                                     }
                                                 );
+  });
 }
 
 RCT_EXPORT_METHOD(getCurrentHighContrastState:(RCTResponseSenderBlock)callback
@@ -161,4 +163,4 @@ - (void)accessibilityDisplayOptionsChange:(NSNotification *)notification
 }
 
 @end
- #endif
+ #endif