Using `StyleSheet.hairlineWidth` in TextInputExample

Summary:
This is workaround for blury and thick borders on iOS when specified border size does not multiplier of pixel size.
Original problem is probably related to CALayer border drawing specifics; documented as T22099662 and
https://github.com/facebook/react-native/issues/14106

Before:
https://pxl.cl/9cJ7

After:
https://pxl.cl/9cJ4

Reviewed By: javache

Differential Revision: D5999752

fbshipit-source-id: ad6d1078c6ebf7c8e0a3bc3c150525480a5a7a5c

RNTester/js/TextInputExample.ios.js

@@ -340,14 +340,14 @@ var styles = StyleSheet.create({
     paddingBottom: 300,
   },
   default: {
-    borderWidth: 0.5,
+    borderWidth: StyleSheet.hairlineWidth,
     borderColor: '#0f0f0f',
     flex: 1,
     fontSize: 13,
     padding: 4,
   },
   multiline: {
-    borderWidth: 0.5,
+    borderWidth: StyleSheet.hairlineWidth,
     borderColor: '#0f0f0f',
     flex: 1,
     fontSize: 13,