Unity3d method to retrieve device ANDROID_ID constant.

AndroidIdRetriever.cs

using UnityEngine;

public static class AndroidIdRetriever
{
    public static string Retrieve()
    {
        AndroidJavaClass clsUnity = new AndroidJavaClass("com.unity3d.player.UnityPlayer");
        AndroidJavaObject objActivity = clsUnity.GetStatic<AndroidJavaObject>("currentActivity");
        AndroidJavaObject objResolver = objActivity.Call<AndroidJavaObject>("getContentResolver");
        AndroidJavaClass clsSecure = new AndroidJavaClass("android.provider.Settings$Secure");
        string ANDROID_ID = clsSecure.GetStatic<string>("ANDROID_ID");
        string androidId = clsSecure.CallStatic<string>("getString", objResolver, ANDROID_ID);
        return androidId;
    }
}