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
26e8426248