Summary:
This replaces ActivityIndicatorIOS and indeterminate ProgressBar that were deprecated recently with ActivityIndicator across the codebase and examples and a few other cleanups.
This also make a small tweak to ActivityIndicator so it uses the Android theme color instead of gray when no color is specified.
Use Slider instead of SliderIOS in CameraRoll example.
Remove the line about unifying ActivityIndicator and ProgressBar.
**Test plan**
Tested the affected components in UIExplorer on iOS and Android, tested the changes made in Movies example on iOS and Android.
Closes https://github.com/facebook/react-native/pull/8082
Differential Revision: D3429770
fbshipit-source-id: 3b2e1196a8b9fe00d47a7aa1bbc079b094796421
4c245160bd