Summary:This adds support for delete view animations in LayoutAnimation for iOS. It supports the same properties as the create animation (alpha, scale).
This allows making simple animations when removing a view which is normally hard to do in React since we need to not remove the view node immediately.
**Test plan**
Tested add/removing views in the UIExample explorer with and without setting a LayoutAnimation. Also tested that the completion callback still works properly. Tested that user interation during the animation is properly disabled.
I also plan to work on improving the doc for LayoutAnimation as well as making this PR for android too.
Closes https://github.com/facebook/react-native/pull/6779
Differential Revision: D3215525
Pulled By: sahrens
fb-gh-sync-id: 526120acd371c8d1af433e8f199cfed336183775
fbshipit-source-id: 526120acd371c8d1af433e8f199cfed336183775
Basil Hosmer
Janic Duplessis
