activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:orientation="vertical"
    tools:context=".MainActivity"
    >

  <TextView
      android:id="@+id/textView"
      android:layout_width="match_parent"
      android:layout_height="136dp"
      android:gravity="center"
      android:text="@string/title"/>

  <Button
      android:id="@+id/qr_scanner_button"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:layout_marginBottom="16dp"
      android:layout_marginEnd="64dp"
      android:layout_marginStart="64dp"
      android:text="@string/qr_button"
      />


</LinearLayout>