Remove UnimplementedView Stubs for iOS and Android Specific Components

[NickGerleman]

We have a number of stubs for iOS and Android specific components which just extend UnimplementView. We shouldn't need to have these, but upstream they are defined as platform specific. E.g. WidgetIOS.android.js instead of WidgetIOS.js. Facebook seemed onboard with making stubs the default in this case, so out-of-tree platforms can consume them. We should do that once on master so we don't need to maintain duplicate stubs.