Summary

Fixes #4477.
Amends PR #4657

I realized that when a Reader theme is selected, accessing the AMP Customizer currently results in Customizer controls et al being registered even for plugins that are suppressed. The reason for this is that suppression was happening at the wp action which is too late (and never even fires) when accessing customize.php. Therefore, this PR fixes that problem by checking if the current request is for an AMP Reader theme (whether on the frontend or the Customizer) and, if so, suppresses plugins immediately at plugins_loaded as early as possible.

This early-suppression could also be done in legacy Reader mode or Transitional mode since there is a query var which indicates it is an AMP request. Nevertheless, this is unnecessary to do because neither Transitional mode nor legacy Reader mode have separate Customizers. Therefore, only “Reader theme” mode needs this early suppression.

To test this:

0. Activate Reader mode and select a Reader theme.
1. Install the latest version of my my Bad Block plugin: https://gist.github.com/westonruter/e4331b3f6d03aac075b86b226acd3ba5
2. Add the Bad block to a post.
3. View the AMP page.
4. Click on Customize to open that AMP URL in the AMP Customizer.
5. Modify the "Bad Block Background Color" in the Colors section.
6. Validate the AMP URL (again).
7. Navigate to AMP settings and suppress Bad Block from running.
8. Navigate back to the AMP page and verify the block is not outputting anything.
9. Open the AMP Customizer for that post and verify the Customizer control for Bad Block Background Color is absent.

Before	After

(The screenshot isn't correct as the Before shouldn't be showing the bad block!)

Checklist

• My pull request is addressing an open issue (please create one otherwise).
• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).