[Android] Rename JNI cpp file

[kirklandsign]

🚀 The feature, motivation and pitch

Would love to have name mapping between jni and java
Rename
extension/android/jni/jni_layer.cpp -> jni_module.cpp
extension/android/jni/jni_layer_llama.cpp -> jni_llm_module.cpp

cc @cbilgin @phaiting Does it make sense?

Alternatives

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Additional context

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RFC (Optional)

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