registration: Improve duplicate config setting exception

registration: Improve duplicate config setting exception

We don't keep track of what set a specific global, so at least mention
the name of the extension that is setting a duplicate for easier
debugging.

Also, fix the case where if the first extension to be loaded was setting
a core setting, it would not throw an exception since config was being
processed before the rest of extension.json. Now we process config after
all core settings, going only before attributes.

Bug: T194319
Change-Id: I4fd96e7d167cf0652ee3e8e66167c86f2b91b992