A test script to read apk assets with JNI.

AndroidReadAsset.cs

using System;
using System.Runtime.InteropServices;
using System.Text;
using UnityEngine;

public class AndroidReadAsset : MonoBehaviour
{
    
#if UNITY_ANDROID && !UNITY_EDITOR
    [DllImport("libc")]
    static extern IntPtr fdopen(int fd, string mode);

    [DllImport("libc")]
    static extern long fread(sbyte[] buffer, long size, long count, IntPtr stream);
    
    [DllImport("libc")]
    static extern long ftello(IntPtr stream);
    
    [DllImport("libc")]
    static extern int fseeko(IntPtr stream, long offset, int whence);

    private IntPtr file;
    private long length;
    private long offset;
    
    // Start is called before the first frame update
    void Start()
    {
        var activity = new AndroidJavaClass("com.unity3d.player.UnityPlayer")
                .GetStatic<AndroidJavaObject>("currentActivity");
        var assetManager = activity.Call<AndroidJavaObject>("getAssets");
        var assetFd = assetManager.Call<AndroidJavaObject>("openFd", "test.bundle");
        var parcelFd = assetFd.Call<AndroidJavaObject>("getParcelFileDescriptor");
        
        length = assetFd.Call<long>("getLength");
        offset = assetFd.Call<long>("getStartOffset");
        var nativeFd = parcelFd.Call<int>("detachFd");
        
        file = fdopen(nativeFd, "rb");
    }

    // Update is called once per frame
    void Update()
    {
        int succ = fseeko(file, offset, 0 /* SEEK_SET */);
        Debug.Log($"fseeko returns {succ}");
        var buffer = new sbyte[11];
        var bytesRead = fread(buffer, (long)1, (long)buffer.Length, file);
        var result = Encoding.UTF8.GetString((byte[])(Array)buffer, 0, (int)bytesRead);
        Debug.Log($"fread returns {result}, read {bytesRead} bytes, current position {ftello(file)}");
    }
#endif
    
}