Signal-Android/contacts/app/src/main/res/layout/activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/contact_list_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Contact List"
        app:layout_constraintBottom_toTopOf="@id/contact_lookup_button"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_chainStyle="packed"/>

    <Button
        android:id="@+id/contact_lookup_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Contact Lookup"
        app:layout_constraintBottom_toTopOf="@id/link_contacts_button"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@id/contact_list_button"
        app:layout_constraintVertical_chainStyle="packed"/>

    <Button
        android:id="@+id/link_contacts_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Link Contacts"
        app:layout_constraintBottom_toTopOf="@id/unlink_contact_button"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@id/contact_lookup_button" />

    <Button
        android:id="@+id/unlink_contact_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Unlink Contacts"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@id/link_contacts_button" />


</androidx.constraintlayout.widget.ConstraintLayout>