rename "Emergency Alert" to "Public Warning" per IT requirements

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

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

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<resources xmlns:android="http://schemas.android.com/apk/res/android" xmlns:xliff="urn:oasis:names:tc:xliff:document:1.2">

    <!-- Cell Broadcast settings title.  [CHAR LIMIT=50] -->

    <string name="cell_broadcast_settings">Public warnings</string>

</resources>

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