ConstraintLayout: how to center a view under another and also stay within parent bounds?

I want to center a view under another like this "sign out" button under the email:

I did this by using a ConstraintLayout as the parent and constraining the bottom view's left and right edges to the left and right edges of the top view. This properly centers the two views. (Note that I don't want to center the views in the parent.)

The problem I'm having is that if the top view is narrow, the bottom view can end up truncated on the right by the parent, as here (from the layout editor):

I don't know in advance how narrow the top view will be at run time. (It won't necessarily be an email address, and, even if it were, I know someone whose email address is only 8 characters!)

I would like to set up constraints so that the bottom view is centered under the top view, but if it ends up too far to the right, it shifts left just enough to avoid crossing a guideline. The right edge of the top view needs to remain fixed. How can I achieve this effect? (I'm not wedded to using a ConstraintLayout if there's another way that will work better.)

Here's the actual layout file I'm using:

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/user_name"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="10dp"
        android:layout_marginTop="10dp"
        android:text="user"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:ignore="HardcodedText"
        tools:layout_constraintRight_creator="1"
        tools:layout_constraintTop_creator="1"/>

    <View
        android:id="@+id/scrim"
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:background="@android:color/transparent"
        android:visibility="gone"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:layout_constraintLeft_creator="1"
        tools:layout_constraintTop_creator="1"
        tools:visibility="visible"/>

    <Button
        android:id="@+id/sign_out"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="10dp"
        android:background="@android:drawable/dialog_holo_light_frame"
        android:text="@string/sign_out"
        android:visibility="gone"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toBottomOf="@id/user_name"
        tools:visibility="visible"/>

</android.support.constraint.ConstraintLayout>