package com.UnityPackage;

import java.io.File;  
import android.os.Environment;  
import android.os.StatFs;

public class UnityTools
{
    public static long GetFreeStorageSpace()
    {
        try
        {
            File file = Environment.getDataDirectory();
            StatFs stat = new StatFs(file.getPath());
            return stat .getAvailableBytes(); //VERSION.SDK_INT >= VERSION_CODES.JELLY_BEAN_MR2
        }
        catch (Exception e)
        {
        }
        return -1;
    }
}

using System;
using System.Collections;
using System.Collections.Generic;
using System.Runtime.InteropServices;
using UnityEngine;

public class UnityAndroidTools
{
    public static long GetFreeStorageSpace()
    {
        try
        {
            AndroidJavaObject javaObj = new AndroidJavaObject("com.UnityPakcage.UnityTools");
            return javaObj.CallStatic<long>("GetFreeStorageSpace");
        }
        catch(Exception e)
        {
            Debug.LogError(e);
        }
        return -1;
    }
}