add privacy manifest to hello world template

Summary:
Changelog: [iOS][Added]

this change will be included in the RN CLI. so all new apps running the RN CLI to get created will get this manifest. the reasons have been added for the following APIs:

NSPrivacyAccessedAPICategoryFileTimestamp
- C617.1: We use fstat and stat in a few places in the C++ layer. We use these to read information about the JavaScript files in RN.

NSPrivacyAccessedAPICategoryUserDefaults
- CA92.1: We access NSUserDefaults in a few places.
1) To store RTL preferences
2) As part of caching server URLs for developer mode
3) A generic native module that wraps NSUserDefaults

NSPrivacyAccessedAPICategorySystemBootTime
- 35F9.1: Best guess reason from RR API pulled in by boost

Reviewed By: cipolleschi

Differential Revision: D53682756

fbshipit-source-id: 0426fe0002a3bc8b45ef24053ac4228c9f61eb85