Site Editor: Provide simple updates to the Customizer experience in AMP theme when Block Themes are active

[ianstewart]

The AMP Plugin in use on WP.com simple sites uses the customizer.

As all other Customizer usage (for the most part) in Block Themes is or should be in the Site Editor, we should take a look making sure that we're not revealing ALL customizer modules to users via the AMP plugin. Perhaps requiring unregistering some things in the Customizer when Block Themes are active or merely hiding the back button.