add simple calculator app

[LuCCoelho]

What does this implement/fix? Explain your changes Simple calculator example

Does this close any currently open issues?

• [x] Yes, Issue #95
• [ ] No

• Where has this been tested? Android A22

• Target Platform: Android

• Demo:

https://github.com/nisrulz/flutter-examples/assets/114317672/4631fc3f-59c4-4181-905d-040330d1c721