Remove schema validation from command parameters/returns

Do not use ObjectSchema and instead pass parameters, progress and
return values as opaque JSON objects.

BUG: 25841230
Change-Id: I0ea5fc31d526b1e5d6c66453b613e7284aa3fcac
Reviewed-on: https://weave-review.googlesource.com/1611
Reviewed-by: Vitaly Buka <vitalybuka@google.com>