Summary:
Simplifies `AppState` by removing redundant methods and changing `addEventListener` to return an `EventSubscription`.
Changelog:
[General][Changed] - `AppState.addEventListener` now returns an `EventSubscription` object.
[General][Removed] - Removed `AppState.removeEventListener`. Instead, use the `remove()` method on the object returned by `AppState.addEventListener`.
[General][Removed] - `AppState` no longer inherits from `NativeEventEmitter`, so it no longer implements `addListener`, `removeAllListeners`, and `removeSubscription`.
Reviewed By: wtfil
Differential Revision: D26161343
fbshipit-source-id: b3cff76bf0f8f7d79cd954fdef551d0654c682ca
Tim Yung
6f22989e92