Migrate UpdateChecker to Kotlin and cleanup unused Java files

• Rewrote UpdateChecker from Java to Kotlin using modern coroutines and sealed classes (update function was broken on previous releases)
• Rewrote VersionInfo as Kotlin data class with property getters
• Updated MainActivity to use new sealed class-based update result handling
• Deleted unused Java files:
  • RpaFileHelper.java (not referenced anywhere)
  • RpyEditorActivity.java (replaced by RpyEditorActivityNew.kt)
  • Test template files (ExampleUnitTest.java, ExampleInstrumentedTest.java)
• Removed old editor layout (activity_rpy_editor.xml)
• Removed old RpyEditorActivity registration from AndroidManifest.xml