Add proto header comments to aconfig files.

This commit adds proto header comments to aconfig files, specifying the
proto file and message type. These comments enable tooling to better
understand and process the aconfig files based on the protobuf schema.
It enables syntax highlighting in code search and provides schema
information to tooling that consumes the files.

See the protobuf documentation for details about header comments:
https://protobuf.dev/reference/protobuf/textformat-spec/#header

Bug: 387193964
Test: Build pass
Flag: EXEMPT refactor
Change-Id: Id442a0d90889166b4c8d78f57e1722afaf09f656