Apple TV Cocoapods support

Summary:
**Motivation**

Support Apple TV for people adding React Native to their projects using Cocoapods.

**Test plan**

Working test project at https://github.com/dlowder-salesforce/react-native-tvos-cocoapods-test
Closes https://github.com/facebook/react-native/pull/15065

Differential Revision: D5443791

Pulled By: javache

fbshipit-source-id: dc46a72df0d73a0049f1c3f9368658e5f3d1ecb8