[ActivityIndicator] Specify a width

Summary:
The activity indicator was treated as a zero-width element without an explicit width. Fill it in so the style dimensions match what is displayed on the screen.

Closes https://github.com/facebook/react-native/pull/1156
Github Author: James Ide <ide@jameside.com>

Test Plan:
 Render an ActivityIndicator with a background, and see that the background shows up as a square behind the spinner instead of not showing up at all (since it was 0px wide previously).

Libraries/Components/ActivityIndicatorIOS/ActivityIndicatorIOS.ios.js

@@ -88,9 +88,11 @@ var styles = StyleSheet.create({
     justifyContent: 'center',
   },
   sizeSmall: {
+    width: 20,
     height: 20,
   },
   sizeLarge: {
+    width: 36,
     height: 36,
   }
 });