bottom_navigation_main.xml

<?xml version="1.0" encoding="utf-8"?>
<menu xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto">
    <item
        android:id="@+id/action_gallery"
        android:enabled="true"
        android:icon="@drawable/ic_photo_library_black_24dp"
        android:title="@string/text_gallery"
        app:showAsAction="ifRoom" />
    <item
        android:id="@+id/action_camera"
        android:enabled="true"
        android:icon="@drawable/ic_camera_alt_black_24dp"
        android:title="@string/text_camera"
        app:showAsAction="ifRoom" />
</menu>

mainscandoc.xml

<?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"
    tools:context=".MainActivity">


    <ImageView
        android:id="@+id/scannedImage"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toTopOf="@+id/bottomNav" />

    <android.support.design.widget.BottomNavigationView
        android:id="@+id/bottomNav"
        android:layout_width="match_parent"
        android:layout_height="60dp"
        android:layout_marginBottom="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:menu="@menu/bottom_navigation_main"
        tools:layout_editor_absoluteX="16dp" />
</android.support.constraint.ConstraintLayout>