Add flow directive to index.android.js

Summary:
iOS template contains flow directive in top comment. Android should too for consistency.
Closes https://github.com/facebook/react-native/pull/7482

Differential Revision: D3281131

fbshipit-source-id: d9a77196faed5e41753d09768721e1da1e6cc60a

local-cli/generator/templates/index.android.js

@@ -1,6 +1,7 @@
 /**
  * Sample React Native App
  * https://github.com/facebook/react-native
+ * @flow
  */
 
 import React, { Component } from 'react';