Amp-wp: Prevent plugin from removing styles for <amp-experiment> variants

[bartesn]

Bug Description

When using <amp-experiment> plugin removes styles that are meant for experiments, for example body[amp-x-experimentName="experimentVariant"] { background: #000; }.
Styles are removed regardless of where they are added (theme's style.css / inline).

Expected Behaviour

Prevent plugin from removing styles that contain an attribute that starts with amp-x- OR add a possibility for adding custom styles that won't be removed by the plugin.

Steps to reproduce

1. Create an experiment on a website with <amp-experiment>
2. Style a variant as in documentation (https://amp.dev/documentation/components/amp-experiment/#style-a-variant) in theme's main stylesheet - style.css.
3. Styles for variants are not being applied.

Additional context

• WordPress version: 5.3.2
• Plugin version: 1.4.4
• AMP plugin template mode: Standard
• PHP version: 7.2.24
• OS: any
• Browser: any
• Device: any

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

@bartesn @JacekKosciesza Please test the fix in this build: https://github.com/ampproject/amp-wp/pull/4438#issuecomment-603444246

[bartesn]

@westonruter I tested the fix and it works as expected. I tested this with styles put in Custom HTML block and custom CSS file. In both cases styles with amp-x-* attribute were applied properly, the plugin doesn't remove them.