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/Components/ActivityIndicator/ActivityIndicatorIOS.android.js

@@ -0,0 +1,22 @@
+/**
+ * Copyright (c) 2015-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @providesModule ActivityIndicatorIOS
+ */
+'use strict';
+
+var React = require('React');
+var View = require('View');
+
+var ActivityIndicatorIOS = React.createClass({
+  render(): ReactElement {
+    return <View {...this.props} />;
+  }
+});
+
+module.exports = ActivityIndicatorIOS;