Implement IsReadyToPay handling

content/public/android/java/src/org/chromium/IsReadyToPayServiceCallback.aidl

+// Copyright 2017 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package org.chromium;
+
+/**
+ * Helper interface to report back whether the
+ * payment app is ready for payment.
+ */
+interface IsReadyToPayServiceCallback {
+    /**
+     * Method to be called by the Service to indicate
+     * whether the payment app is ready for payment.
+     *
+     * @param isReadyToPay Whether payment app is ready to pay.
+     */
+    oneway void handleIsReadyToPay(boolean isReadyToPay);
+}