TestCallback.cs

using System;
using UnityEngine;

class TestCallback {
  public static AndroidJavaObject apply<T1, T2>(Action<T1, T2> action) {
    var cb = new AndroidJavaObject("TestCallback");
    cb.Call("setRunnable", new AndroidJavaRunnable(() => {
      var p1 = cb.Call<T1>("getParam1");
      var p2 = cb.Call<T2>("getParam2");
      action.Invoke(p1, p2);
    }));
    return cb;
  }
}

TestCallback.java

public class TestCallback<T1, T2> {
  private Runnable runnable;
  private T1 param1;
  private T2 param2;

  public void setRunnable(Runnable runnable) {
    this.runnable = runnable;
  }

  public void call(T1 param1, T2 param2) {
    this.param1 = param1;
    this.param2 = param2;
    runnable.run();
  }

  public T1 getParam1() {
    return param1;
  }

  public T2 getParam2() {
    return param2;
  }
}