registration: Only allow one extension to set a specific config setting

ExtensionProcessor would previously just blindly overwrite duplicate
config settings, which ends up depending upon load order.

It's relatively hard to debug since it is silently overwritten. This now
throws exceptions in case of duplicate config settings.

This will also have some side-effects of catching people putting things
like "ResourceModules" in their "config" section when it should be a
top-level item.

Bug: T152929
Depends-On: I4c5eaf87657f5dc07787480a2f1a56a1db8c714f
Change-Id: Ieeb26011e42c741041d2c3252238ca0823b99eb4