Summary:
Enable back navigation on Apple TV (with the remote's menu button) in code making use of BackAndroid. The module is renamed to BackHandler. BackAndroid is still exported to ReactNative for now, until external projects switch to using the new name for the module. The navigation in https://github.com/react-community/react-navigation makes use of this module.
**Test plan**: Manual testing with an example app (https://github.com/dlowder-salesforce/react-nav-example).
Closes https://github.com/facebook/react-native/pull/12571
Differential Revision: D4665152
Pulled By: ericvicenti
fbshipit-source-id: 925400ce216379267e014457be6f5eedbe4453ec
Douglas Lowder