Refactor Android implementation to use ContentObserver

[foxanna]

Fix for #29

Inspired by ScreenshotDetector by @nikit19.

[Ascenio]

Isn't it a bad practice to use multiple languages in the same project? I mean, it works but the devs have to know both Java and Kotlin. Also I've noticed that you renamed private variables to use the _foo notation, that works but it's not so idiomatic of Java, it's more a darty thing.

[foxanna]

Hi @Ascenio,
Thanks for the comments! I myself would be more than happy to see how the ideal implementation should look like.
I had a need to detect screenshots in the Flutter app and had fixed this plugin with the solution I could find. It's far from perfect but it works, so I thought I'd share it for anyone in need of an Android fix.
If you or anyone is interested in improving the solution, feel free to use this as a starting point. Best of luck!

[beygee]

It has been modified to reflect your request.
I apologize for not being able to merge directly due to a conflict.
Thank you for your hard work!
https://pub.dev/packages/screenshot_callback/changelog