Update AppState Jest Mock

Summary:
Updates the AppState Jest mock to correspond to recent API changes.

Changelog:
[General][Fixed] - Updated AppState Jest mock to match new public API

Reviewed By: yungsters

Differential Revision: D26295371

fbshipit-source-id: e6e39f23d9100f0eacf257f45d509c5364dcb28d

jest/setup.js

@@ -160,8 +160,9 @@ jest
     ),
   )
   .mock('../Libraries/AppState/AppState', () => ({
-    addEventListener: jest.fn(),
-    removeEventListener: jest.fn(),
+    addEventListener: jest.fn(() => ({
+      remove: jest.fn(),
+    })),
   }))
   .mock('../Libraries/Linking/Linking', () => ({
     openURL: jest.fn(),