Summary:
Simplifies `Keyboard` by removing redundant methods and changing `addEventListener` to return an `EventSubscription`.
Changelog:
[General][Changed] - `Keyboard.addListener` now returns an `EventSubscription` object.
[General][Removed] - Removed `Keyboard.removeListener`. Instead, use the `remove()` method on the object returned by `Keyboard.addListener`.
[General][Removed] - `Keyboard` no longer inherits from `NativeEventEmitter`, so it no longer implements `removeAllListeners`, and `removeSubscription`.
Reviewed By: milroc
Differential Revision: D26163536
fbshipit-source-id: b4bd91627cd027a13fcba269a253823913eb7589