Restructure Kotlin APIs to Share Common Code in core/

[rbebb]

Summary

The goal of this is to prep the Kotlin APIs for eventual Kotlin Multiplatform support. For now, that just means that common code is shared in core/.

Also, the Kotlin rules for Bazel don’t support Kotlin Multiplatform yet (see this GitHub Issue for updates) so that's why I didn't use Kotlin Multiplatform's expect/actual syntax. Once the Kotlin rules for Bazel do add support, the code can be simplified a bit.

Changes

• Created /core package that can be built separately
• Changed /kotlin/java/ directory to /kotlin/kotlin/
• Made NativeLibraryLoader and Engine abstract classes with platform specific implementations
• Marked platform specific Engine implementations as internal but kept the API to create and use an Engine the same
• Changed LiteRtLmJniException to LiteRtLmNativeException to better reflect the intent for eventual multiplatform support
• Replaced some JVM APIs with Kotlin APIs

Questions/Comments

• This is my first PR for LiteRT-LM
  • Is there anything about PR and commit formatting that should be changed?
• I'm open to any and all feedback on the implementation!

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[rbebb]

I assume a -core package won't be published just yet, so this multiplatform section might not be necessary for now.