Add Image#getSize for Android

Summary:
I've implemented the getSize method on Image for Android.

**Test plan (required)**

The result in the UIExample app can be seen here:

![android-getsize](https://cloud.githubusercontent.com/assets/570297/15442613/a29c9178-1ee2-11e6-97df-adc20aad0c32.jpg)
Closes https://github.com/facebook/react-native/pull/7664

Differential Revision: D3331704

fbshipit-source-id: d784c861cbc653cd6b49310f4b5516c6583486ca

Examples/UIExplorer/ImageExample.js

@@ -509,7 +509,6 @@ exports.examples = [
     render: function() {
       return <ImageSizeExample source={fullImage} />;
     },
-    platform: 'ios',
   },
 ];