Consolidate XHR UIExplorer examples

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The XHRExample file had been forked between iOS and Android for various reasons that are no longer valid, making maintenance unnecessarily difficult.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