Centering controls in a screen - can't do it

How do you force controls to be centered inside a view?
Controls inside a vertical layout inside a horizontal layout wont do it
Setting android:layout_centerHorizontal="true" on either layout won't do it.
Setting 'android:layout_centerParent="true" on either layout.
Setting 'match_parent' on either layout won't do it.
If I try and replace '365dp' in android:layout_width="395dp" with 'match_parent' then irritatingly android studio just overrides what I type and replaces it with 395dp again.
I just don't understand any of this.
What do I have to do to get android studio to centre my controls both vertically and horizontally?

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

xmlns:app=""
xmlns:tools=""
android:id="@+id/id_view_main"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical"
tools:context="com.example.greg.irrigationcontroller.MainActivity">

<LinearLayout
    android:layout_width="395dp"
    android:layout_height="587dp"
    android:layout_centerHorizontal="true"
    android:orientation="horizontal">

    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:orientation="vertical">

        <TextView
            android:id="@+id/id_text_prompt"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_marginTop="0dp"
            android:text="What would you like to do? "
            android:textAppearance="@android:style/TextAppearance.Material.Medium"
            android:textColor="@color/colorAccent"
            android:textSize="24sp"
            android:textStyle="bold"
            app:layout_constraintHorizontal_bias="0.672"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toTopOf="parent" />

        <Button
            android:id="@+id/id_button_settings"
            android:layout_width="300dp"
            android:layout_height="40dp"
            android:onClick="SendMessage"
            android:text="Change watering settings"
            app:layout_constraintHorizontal_bias="0.786"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            tools:layout_editor_absoluteY="374dp" />

        <Button
            android:id="@+id/id_button_show"
            android:layout_width="300dp"
            android:layout_height="40dp"
            android:text="Show station settings"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintHorizontal_bias="0.297"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintVertical_bias="0.479" />

        <Button
            android:id="@+id/id_button_search"
            android:layout_width="300dp"
            android:layout_height="40dp"
            android:text="Search for active stations"
            app:layout_constraintHorizontal_bias="0.912"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            tools:layout_editor_absoluteY="326dp" />
    </LinearLayout>
</LinearLayout>