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

Libraries/Image/Image.android.js

@@ -113,6 +113,21 @@ var Image = React.createClass({
 
   statics: {
     resizeMode: ImageResizeMode,
+
+    getSize(
+      url: string,
+      success: (width: number, height: number) => void,
+      failure: (error: any) => void,
+    ) {
+      return ImageLoader.getSize(url)
+        .then(function(sizes) {
+          success(sizes.width, sizes.height);
+        })
+        .catch(failure || function() {
+          console.warn('Failed to get size for image: ' + url);
+        });
+    },
+
     /**
      * Prefetches a remote image for later use by downloading it to the disk
      * cache