Fix crash caused by ggml_backend_load_all when launching on Android Activity (llama...

Fix crash caused by ggml_backend_load_all when launching on Android Activity (llama/10812)

* Fix crash caused by ggml_backend_load_all when launching on AndroidActivity.

Details:
Calling ggml_backend_load_all during initialization in the AndroidActivity project leads to a crash with the error:
terminating with uncaught exception of type std::__ndk1::__fs::filesystem::filesystem_error: filesystem error: in directory_iterator::directory_iterator(...): Permission denied [./].
This issue occurs because AndroidActivity restricts file access due to sandboxing.

Reproduction:
In the example folder, the LlamaAndroid project can reproduce the crash by calling ggml_backend_load_all first in Java_android_llama_cpp_LLamaAndroid_backend_1init.

* Update ggml/src/ggml-backend-reg.cpp

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Co-authored-by: Diego Devesa <redacted>