Refactor java to kotlin, add showInAppMessageAndroid

[offline-first]

-Refactor java to kotlin
-Improve android implementation
-Update Billing library to 4.1.0
-Add support for in app messages on android (Play Store)

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[codecov]

Codecov Report

Merging #365 (e2c41a0) into main (0c7baba) will decrease coverage by 3.56%.
The diff coverage is 11.36%.

@@            Coverage Diff             @@
##             main     #365      +/-   ##
==========================================
- Coverage   45.74%   42.18%   -3.57%     
==========================================
  Files           3        4       +1     
  Lines         435      467      +32     
==========================================
- Hits          199      197       -2     
- Misses        236      270      +34     

Impacted Files	Coverage Δ
lib/modules.dart	31.76% <0.00%> (ø)
lib/flutter_inapp_purchase.dart	46.95% <9.75%> (-7.08%)	⬇️
lib/Store.dart	100.00% <100.00%> (ø)

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[soarescaique]

This PR seems to fix Issue #363 . Got no errors testing.

[hyochan]

Thanks for reviewing this. Since these are big changes I hope more people to spend time on this PR so that we can prevent critical regressions 🙏

[offline-first]

I use this Version since one week in production without any issues! The next commits are smaller :)

[soarescaique]

I have reviewed the changes this week, and they are looking good! Just waiting for the merge to launch it to production

[hyochan]

I've not actually reviewed this yet.
However, there are people who want this PR to be merged so I'll give a try.

Hope it is safe.

Thanks again for the great work 🙏