[LayoutAnimation] RCTAnimationTypeKeyboard

Summary:
This adds the Keyboard animation type for when you want to animate UI based on the keyboard appearing/disappearing.
Closes https://github.com/facebook/react-native/pull/1366
Github Author: Stanislav Vishnevskiy <vishnevskiy@gmail.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Libraries/Animation/LayoutAnimation.js

@@ -23,6 +23,7 @@ var TypesEnum = {
   easeInEaseOut: true,
   easeIn: true,
   easeOut: true,
+  keyboard: true,
 };
 var Types = keyMirror(TypesEnum);