Prevent non-legacy themes from being selected as Reader theme on Settings screen when AMP slug defined late

[westonruter]

Bug Description

In #4999 we check if the AMP slug (query var) is defined too late and if so, prevent selection of a non-legacy Reader theme:

This, however, was not likewise implemented for the Settings screen which allows all to be selected:

Expected Behaviour

The available themes should be consistent between the Settings screen and the Onboarding Wizard.

Steps to reproduce

Activate this plugin:

<?php
/**
 * Plugin Name: AMP Define Query Var Early with Constant
 */

add_action(
	'after_setup_theme',
	function () {
		add_filter(
			'amp_query_var',
			function() {
				return 'too-early-with-constant';
			}
		);
	},
	~PHP_INT_MAX
);

Then access the AMP Settings screen and select Reader template mode.

This is part of #4795.

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

QA Passed

To test this, I added a plugin that did this:

add_action( 'setup_theme', function () {
	define( 'AMP_QUERY_VAR', 'lite' );
} );

When doing so, the settings screen shows:

And the Onboarding Wizard:

So the non-legacy Reader themes are indeed prevented from being used.