Set overlayable for CarsystemUI NavBar

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

<!-- Copyright (C) 2020 The Android Open Source Project

     Licensed under the Apache License, Version 2.0 (the "License");

     you may not use this file except in compliance with the License.

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<resources>

    <overlayable name="SystemBarsLayouts">

        <policy type="product|signature">

            <item type="layout" name="car_navigation_bar" />

        </policy>

    </overlayable>

</resources>

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