packages/SystemUI/res/layout/smart_reply_view.xml

<?xml version="1.0" encoding="utf-8"?>

<!--
  ~ Copyright (C) 2017 The Android Open Source Project
  ~
  ~ Licensed under the Apache License, Version 2.0 (the "License");
  ~ you may not use this file except in compliance with the License.
  ~ You may obtain a copy of the License at
  ~
  ~      http://www.apache.org/licenses/LICENSE-2.0
  ~
  ~ Unless required by applicable law or agreed to in writing, software
  ~ distributed under the License is distributed on an "AS IS" BASIS,
  ~ WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  ~ See the License for the specific language governing permissions and
  ~ limitations under the License
  -->

<!-- LinearLayout -->
<com.android.systemui.statusbar.policy.SmartReplyView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:systemui="http://schemas.android.com/apk/res-auto"
    android:id="@+id/smart_reply_view"
    android:layout_height="wrap_content"
    android:layout_width="wrap_content"
    systemui:spacing="@dimen/smart_reply_button_spacing"
    systemui:singleLineButtonPaddingHorizontal="@dimen/smart_reply_button_padding_horizontal_single_line"
    systemui:doubleLineButtonPaddingHorizontal="@dimen/smart_reply_button_padding_horizontal_double_line">
    <!-- smart_reply_button(s) will be added here. -->
</com.android.systemui.statusbar.policy.SmartReplyView>