Summary:
The XHRExample file had been forked between iOS and Android for various reasons that are no longer valid, making maintenance unnecessarily difficult. New features demos had in fact been added in separate files to avoid further duplication. This change continues on that same trajectory and splits the entire example file into multiple files. This makes them much easier to handle. We also get rid of the iOS/Android fork.
**Test plan:** Run UIExplorer app, specifically the XHR examples, on both iOS and Android.
Closes https://github.com/facebook/react-native/pull/11901
Differential Revision: D4419882
Pulled By: ericvicenti
fbshipit-source-id: ec12836dfa8e1b9259b92a7510914857a8db58d5
Philipp von Weitershausen