Cross platform ActivityIndicator

Summary:
The API for `ActivityIndiatorIOS` and `ProgressBarAndroid` is very similar and can be merged in a cross platform component that displays a circular indeterminate loading indicator.

This deprecates `ActivityIndiatorIOS` and non-horizontal `ProgressBarAndroid` in favor of this new component.

**Test plan (required)**

Tested with the ActivityIndicator example in UIExplorer on android and ios. Also made sure that `ActivityIndicatorIOS` still works and displays a deprecation warning. Also tested that `ProgressBarAndroid` with `indeterminate == true` and `styleAttr != 'Horizontal'` displays a deprecation warning.
Closes https://github.com/facebook/react-native/pull/6897

Differential Revision: D3351607

Pulled By: dmmiller

fbshipit-source-id: b107ce99d966359003e8b3118cd97b90fa1d3d7d

Libraries/react-native/react-native.js

@@ -27,6 +27,7 @@ if (__DEV__) {
 // Export React, plus some native additions.
 const ReactNative = {
   // Components
+  get ActivityIndicator() { return require('ActivityIndicator'); },
   get ActivityIndicatorIOS() { return require('ActivityIndicatorIOS'); },
   get ART() { return require('ReactNativeART'); },
   get DatePickerIOS() { return require('DatePickerIOS'); },

Libraries/react-native/react-native.js.flow

@@ -25,6 +25,7 @@
 //
 var ReactNative = Object.assign(Object.create(require('ReactNative')), {
   // Components
+  ActivityIndicator: require('ActivityIndicator'),
   ActivityIndicatorIOS: require('ActivityIndicatorIOS'),
   ART: require('ReactNativeART'),
   DatePickerIOS: require('DatePickerIOS'),