Convert remaining two system hooks to OOP

I might split out the page attachments from system_theme

Page attachments is pretty straightforward.

system_theme is getting a bit complicated I fixed the install process, RegistryTest will likely fail still.

I imagine it needs some discussion.

The root of the issue for system_theme is that when Registry is called during install processExtension doesn't register system_theme as a hook when it's an object, but it does for the function even though the invoke call doesn't use the legacyInvoke fallback.

I then moved it to a static method so I could add it when needed, and combined system_theme and drupal_common_theme since that was the only call to drupal_common_theme (though that might be like that to load the constants in theme.inc)

I'm going to split system_theme out of this cause that has gone a bit far afield.

nicxvan → changed the visibility of the branch 3487971-convert-remaining-two to hidden.

Potentially this will unblock #2719315: Try to install system module like any other module → or at least get us part of the way there.

When I've looked at this problem for #2719315: Try to install system module like any other module → and similar issues I've repeatedly got stuck on wondering what really needs to be added from the BareHtmlPageRenderer vs. what could possibly only be added by system_page_attachments() - but just factoring out the entire method as a start makes sense and avoids getting stuck on what could potentially live where.

A couple small comments.

Applied three and responded to one comment.

lgtm