Change appearance of ActivityIndicator

This looks more like a traditional OS-level activity indicator.

The design of the Android activity indicator hasn't worked very well for
us on Web. The main problem is that if the main thread is locked, the
indicator (even if it's a gif) will stop animating and can look really
bad. This implementation looks like an activity indicator even when it
isn't animating.

examples/components/ActivityIndicator/ActivityIndicatorExample.js

@@ -59,12 +59,11 @@ const ToggleAnimatingActivityIndicator = React.createClass({
 
 const examples = [
   {
-    title: 'Default (small, white)',
+    title: 'Default',
     render() {
       return (
         <ActivityIndicator
-          style={[styles.centering, styles.gray]}
-          color="white"
+          style={[styles.centering]}
         />
       );
     }
@@ -78,7 +77,7 @@ const examples = [
             style={[styles.centering]}
           />
           <ActivityIndicator
-            style={[styles.centering, {backgroundColor: '#eeeeee'}]}
+            style={[styles.centering, styles.gray]}
           />
         </View>
       );