Summary:
**Motivation**
Resolves issue: #12367. The issue is that the packager does not launch automatically on Windows when running `react-native run-android` like it does on OSX. The proposed change fixes this issue, and causes the packager to be launched automatically on Windows 10. Also note the updated code uses the syntax described in the [Node.js documentation](https://nodejs.org/api/all.html#child_process_spawning_bat_and_cmd_files_on_windows) on spawning .bat files on Windows.
**Test plan**
Manually tested on Windows 10.
Running just `react-native run-android` now results in the packager launching, without needing to run `react-native start` in a new command prompt window.
**Screenshot:**
Closes https://github.com/facebook/react-native/pull/12755
Differential Revision: D4767321
fbshipit-source-id: a14f369ba99939aa44d0ee3403e1d262e2657e6e
f891985bd8
