Use template_dir consistently as signal for Transitional mode

[westonruter]

An issue reported by @archon810 was that if if a theme contains a theme support declaration like the following:

add_theme_support( 'amp', [
	'template_dir' => 'amp',
] );

The theme support option in the admin will default to Standard when first activating the plugin, as opposed to the Transitional mode, which is what is implied by supplying template_dir without paired. This is shown in amp_is_canonical():

amp-wp/amp.php

Lines 600 to 601 in 93276f7

	// If there is a template_dir, then transitional mode is implied.
	return empty( $args['template_dir'] );

The issue is that this same logic is not being used AMP_Theme_Support::read_theme_support():

amp-wp/includes/class-amp-theme-support.php

Line 321 in 93276f7

$is_paired = ! empty( $args[ self::PAIRED_FLAG ] );

This is something that was likely missed in #2550.

So this PR ensures that AMP_Theme_Support::read_theme_support() results in the same theme support being derived as is indicated by amp_is_canonical().

To test this PR:

1. Add the above add_theme_support() to a theme's functions.php
2. Run wp option delete amp-options.
3. Load the AMP settings screen.
4. You should see Transitional as the selected mode as opposed to Standard.

[kienstra]

Approved

Hi @westonruter,
This looks good.

Like you mentioned in your testing instructions, 'Transitional' now appears by default in 'AMP Settings' if the theme calls amp_theme_support( 'amp' ) with a template_dir.

Before

After

[kienstra]

Great testing of this.

[westonruter]

Moved directly to Ready for Merging.