Fixed ImageLoader Mock for Android

Summary:
When trying to mock an Image on Android, this error occurs referencing the ImageLoader mock in setup.js:
```
/Users/.../node_modules/react-native/jest/setup.js:120
return success(320,240);
       ^

TypeError: success is not a function
    at /Users/.../node_modules/react-native/jest/setup.js:120:8
    at _combinedTickCallback (internal/process/next_tick.js:73:7)
    at process._tickCallback (internal/process/next_tick.js:104:9)
```
- Mocking Images works for iOS but not Android.
- Since Image.android.js uses ImageLoader vs ImageViewManager, the code cannot be carried over from iOS.

Since this is code within the testing framework, the existing tests should test this. If there is no existing test coverage, then this would need to be done.
Closes https://github.com/facebook/react-native/pull/14262

Differential Revision: D5162640

Pulled By: shergin

fbshipit-source-id: 5b1efbbb685b767e09b5c7fb05f073445c94a3aa

jest/setup.js

@@ -124,7 +124,7 @@ const mockNativeModules = {
   },
   ImageLoader: {
     getSize: jest.fn(
-      (uri, success) => process.nextTick(() => success(320, 240))
+      (url) => new Promise(() => ({width: 320, height: 240}))
     ),
     prefetchImage: jest.fn(),
   },