Provide example of android side implementation of HeadlessJS
Provide example of android side implementation of HeadlessJS

Summary:
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- [x] Explain the **motivation** for making this change.
- [ ] Provide a **test plan** demonstrating that the code is solid.
- [ ] Match the **code formatting** of the rest of the codebase.
- [ ] Target the `master` branch, NOT a "stable" branch.

The existing documentation on this topic doesn't provide enough of detail on where to start a headlessJS task.

Just docs change

This PR extends HeadlessJS documentation to show an example on how to start HeadlessJsTaskService. Sample code in example reacts to connectivity change using custom broadcast receiver logic that bundles additional information and starts the aforementioned service.
Closes https://github.com/facebook/react-native/pull/13957

Differential Revision: D5746809

Pulled By: hramos

fbshipit-source-id: 3eeea7d4a71382acf4b6f7ad1b99d20e2745c558

pandiaraj44/react-native,myntra/react-native,hammerandchisel/react-native,ptmt/react-native-macos,hoastoolshop/react-native,ptmt/react-native-macos,hoangpham95/react-native,kesha-antonov/react-native,hoangpham95/react-native,facebook/react-native,myntra/react-native,exponent/react-native,exponent/react-native,Bhullnatik/react-native,exponent/react-native,Bhullnatik/react-native,ptmt/react-native-macos,rickbeerendonk/react-native,kesha-antonov/react-native,exponentjs/react-native,kesha-antonov/react-native,arthuralee/react-native,dikaiosune/react-native,facebook/react-native,frantic/react-native,hoastoolshop/react-native,hoangpham95/react-native,myntra/react-native,hammerandchisel/react-native,jevakallio/react-native,pandiaraj44/react-native,hoastoolshop/react-native,jevakallio/react-native,jevakallio/react-native,rickbeerendonk/react-native,exponent/react-native,pandiaraj44/react-native,jevakallio/react-native,ptmt/react-native-macos,cdlewis/react-native,arthuralee/react-native,exponentjs/react-native,kesha-antonov/react-native,rickbeerendonk/react-native,exponent/react-native,cdlewis/react-native,dikaiosune/react-native,hammerandchisel/react-native,Bhullnatik/react-native,frantic/react-native,cdlewis/react-native,hammerandchisel/react-native,exponentjs/react-native,Bhullnatik/react-native,arthuralee/react-native,exponent/react-native,cdlewis/react-native,arthuralee/react-native,exponentjs/react-native,rickbeerendonk/react-native,javache/react-native,javache/react-native,hoangpham95/react-native,janicduplessis/react-native,ptmt/react-native-macos,jevakallio/react-native,exponentjs/react-native,jevakallio/react-native,janicduplessis/react-native,exponent/react-native,kesha-antonov/react-native,janicduplessis/react-native,ptmt/react-native-macos,cdlewis/react-native,facebook/react-native,hoastoolshop/react-native,javache/react-native,kesha-antonov/react-native,myntra/react-native,exponentjs/react-native,rickbeerendonk/react-native,hammerandchisel/react-native,hoangpham95/react-native,exponentjs/react-native,exponent/react-native,javache/react-native,Bhullnatik/react-native,facebook/react-native,rickbeerendonk/react-native,facebook/react-native,javache/react-native,dikaiosune/react-native,cdlewis/react-native,janicduplessis/react-native,kesha-antonov/react-native,frantic/react-native,hoastoolshop/react-native,javache/react-native,hammerandchisel/react-native,exponentjs/react-native,jevakallio/react-native,facebook/react-native,hammerandchisel/react-native,myntra/react-native,javache/react-native,hoastoolshop/react-native,javache/react-native,dikaiosune/react-native,arthuralee/react-native,hoangpham95/react-native,janicduplessis/react-native,pandiaraj44/react-native,Bhullnatik/react-native,hoastoolshop/react-native,janicduplessis/react-native,ptmt/react-native-macos,facebook/react-native,cdlewis/react-native,jevakallio/react-native,pandiaraj44/react-native,rickbeerendonk/react-native,pandiaraj44/react-native,myntra/react-native,facebook/react-native,ptmt/react-native-macos,frantic/react-native,frantic/react-native,janicduplessis/react-native,janicduplessis/react-native,rickbeerendonk/react-native,dikaiosune/react-native,myntra/react-native,hoangpham95/react-native,rickbeerendonk/react-native,pandiaraj44/react-native,hammerandchisel/react-native,Bhullnatik/react-native,Bhullnatik/react-native,dikaiosune/react-native,hoangpham95/react-native,cdlewis/react-native,kesha-antonov/react-native,dikaiosune/react-native,myntra/react-native,dikaiosune/react-native,jevakallio/react-native,kesha-antonov/react-native,myntra/react-native,hoastoolshop/react-native,cdlewis/react-native,pandiaraj44/react-native,javache/react-native,facebook/react-native