javascriptInterfaceAddCustomer.cs

 public class AddCustomerInterfacing : Java.Lang.Object, Java.Lang.IRunnable
    {
        private Context context;
        private Activity _activity;
        public const int RQS_PICK_CONTACT = 1;

        public AddCustomerInterfacing(Context context, Activity activity)
        {
            this.context = context;
            _activity = activity;
        }

        // Android API 17 and above 
        [Export]
        [JavascriptInterface]
        public void addCustomer()
        {
            var contactIntent = new ContactUtil().CreatePhoneLookupIntent();
            _activity.StartActivityForResult(contactIntent, RQS_PICK_CONTACT);
        }

        /// <summary>
        /// Add support for API 16 below
        /// </summary>
        public void Run()
        {
            addCustomer();
        }
        
    }