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

Examples/UIExplorer/UIExplorerList.android.js

@@ -23,6 +23,10 @@ export type UIExplorerExample = {
 };
 
 var ComponentExamples: Array<UIExplorerExample> = [
+  {
+    key: 'ActivityIndicatorExample',
+    module: require('./ActivityIndicatorExample'),
+  },
   {
     key: 'SliderExample',
     module: require('./SliderExample'),