registration: Convert "config" into an object with metadata

registration: Convert "config" into an object with metadata

To add extra metadata for "config" options without constantly adding
hacky underscore prefixed keys, convert "config" items into an object,
where the value is under a "value" key.

"_merge_strategy" is now just "merge_strategy", but still has the
underscore prefix for the cache structure, to avoid changing it.

Since this is a fully breaking change, it only applies to files with
manifest_version: 2 set. A conversion script has been added to assist
with automated conversion.

Bug: T133626
Change-Id: Id1071fc0647892438e5cd0e3ee621fbdaaa64014