Convert AppConstant.java into kotlin

Description

Small refactoring MR, coming from forgiven MR for issue !237 (closed).

Screenshots

Technical details

I try to keep this MR atomic

• Transform java class into kotlin
• Remove unused and useless constant
• Replace one import with * by the required class name.

Tests

Issues

• https://gitlab.e.foundation/e/os/backlog/-/issues/1246

10 commandments of code reviews

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Summary by CodeRabbit

• New Features

  • Introduced a centralized repository for application constants to improve maintainability and accessibility.

• Bug Fixes

  • Enhanced clarity of constant names across multiple classes by adopting consistent naming conventions.

• Tests

  • Updated test cases to reflect new constant names for better clarity and consistency.
  • Minor adjustments made to streamline imports in test files.