Add center ImageResizeMode option

Summary:Adds a `center` option to `Image`'s `resizeMode` prop, which doesn't enlarge images.
This is how it looks in UIExplorer:
{F60386921}

Reviewed By: dmmiller

Differential Revision: D3064284

fb-gh-sync-id: 79cd2da8f44c5b3da2e42d3bebf3131335f53c28
shipit-source-id: 79cd2da8f44c5b3da2e42d3bebf3131335f53c28

ReactAndroid/src/main/java/com/facebook/react/views/image/ImageResizeMode.java

@@ -33,6 +33,9 @@ public class ImageResizeMode {
     if ("stretch".equals(resizeModeValue)) {
       return ScalingUtils.ScaleType.FIT_XY;
     }
+    if ("center".equals(resizeModeValue)) {
+      return ScalingUtils.ScaleType.CENTER_INSIDE;
+    }
     if (resizeModeValue == null) {
       // Use the default. Never use null.
       return defaultValue();