Summary: public
UIActionSheet no longer works on iPad when building with the iOS 9 SDK, as reported here: https://github.com/facebook/react-native/issues/3498
This diff implements an alernative code path that uses UIAlertController instead when its available.
Also added `anchor` support for action sheets (previously only supported for share sheet).
Reviewed By: javache
Differential Revision: D2560068
fb-gh-sync-id: feda20f6f1fee5e2ba624761fbeb812e23d96fff
a859d9e931