ConnectionCallbacks.cs

using System;
using UnityEngine;

namespace com.tinylabproductions.u3d_gps_bridge {
  public class ConnectionCallbacks : AndroidJavaProxy {
    public ConnectionCallbacks() : 
    base("com.tinylabproductions.u3d_gps_bridge.ConnectionCallbacks")
    {}

    public Action OnConnected = delegate {};
    public Action OnDisconnected = delegate {};
    // Params: errorCode
    public Action<int> OnConnectionFailed = delegate { };

    /** Java interface methods **/

    void onConnected() { OnConnected.Invoke(); }
    void onDisconnected() { OnDisconnected.Invoke(); }
    void onConnectionFailed(int errorCode) {
      OnConnectionFailed.Invoke(errorCode);
    }
  }
}

ConnectionCallbacks.java

package com.tinylabproductions.u3d_gps_bridge;

public interface ConnectionCallbacks {
  public void onConnected();
  public void onDisconnected();
  public void onConnectionFailed(int errorCode);
}