Activity indicator: add size prop

Summary:
**motivation**

Previously, size can only accept either 'small' or 'large'. And to obtain a custom size, scale transformation is used. This is to let users to possibly pass number value directly to define ActivityIndicator's size.

**Test plan**

I have also modified the current example to reflect the new size prop in action.
Closes https://github.com/facebook/react-native/pull/8935

Differential Revision: D3637910

fbshipit-source-id: 6b8e1d4504964916df327b2d3eaaef1bb8cd5112

Examples/UIExplorer/js/ActivityIndicatorExample.js

@@ -113,8 +113,8 @@ exports.examples = [
       return (
         <ActivityIndicator
           style={[styles.centering, styles.gray]}
-          color="white"
           size="large"
+          color="white"
         />
       );
     }
@@ -161,8 +161,21 @@ exports.examples = [
       );
     }
   },
+  {
+    platform: 'android',
+    title: 'Custom size (size: 75)',
+    render() {
+      return (
+        <ActivityIndicator
+          style={styles.centering}
+          size={75}
+        />
+      );
+    }
+  },
 ];
 
+
 const styles = StyleSheet.create({
   centering: {
     alignItems: 'center',