Add example for focus and blur events to AppStateExample (#30381)

Summary:
Added an example to test focus and blur events in AppState (Android only)

## Changelog

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[Android] [Added] - Add example for focus and blur events to AppStateExample

Pull Request resolved: https://github.com/facebook/react-native/pull/30381

Test Plan: ![Example](http://g.recordit.co/6FGnSzKQaG.gif)

Reviewed By: cpojer

Differential Revision: D25245334

Pulled By: rickhanlonii

fbshipit-source-id: 916f18a74140cc43e0d513eb4073cbcc44e41427