registration: Move attributes out of the top level

registration: Move attributes out of the top level

This moves attributes out of the top level, and namespaces them under
each extension. If the extension that it belongs to is not installed,
the attribute is not exported and dropped.

The full name of the attribute is the name of the extension plus the
name of the attribute key. This enforces the recommendation that the
attribute name start with the extension's name.

Add test coverage for attributes under manifest_version 1 and 2.

Bug: T133627
Depends-On: I5a148763f68989c8da313a4fb1d0213658ee4495
Depends-On: I5a148763f68989c8da313a4fb1d0213658ee4459
Change-Id: I8613a027c56e2c9d2c6a83ca14749eb1c8fc23be