Fix AccessibilityInfo mock to always return a promise in isScreenReaderEnabled

Summary:
`AccessibilityInfo.isScreenReaderEnabled().then()` fails by default in tests because that function is mocked and it doesn't return a promise (the default return value is `undefined`). Returning `Promise.resolve(false)` is a good default in this case so it doesn't break any code relying on it.

Changelog: [General] [Fixed] - Fix `AccessibilityInfo.isScreenReaderEnabled` mock in Jest setup

Reviewed By: lunaleaps, motiz88

Differential Revision: D19972921

fbshipit-source-id: 3c8498d5aeeac54d1f5cf333ae39658e22c180a1

Author: rubennorte

jest/setup.js

@@ -109,7 +109,7 @@ jest
     isInvertColorsEnabled: jest.fn(),
     isReduceMotionEnabled: jest.fn(),
     isReduceTransparencyEnabled: jest.fn(),
-    isScreenReaderEnabled: jest.fn(),
+    isScreenReaderEnabled: jest.fn(() => Promise.resolve(false)),
     removeEventListener: jest.fn(),
     setAccessibilityFocus: jest.fn(),
   }))