Remove source as a publisher argument

We used to have source as a publisher argument, but this anyway ends being
included in its Sample itself. So rather than complicating the internal
publishing API, simplify it and just include source as a Sample field.

Change-Id: Idb9d0c2b5969318dd16394a81d7d61c67f613892
rpc: remove source argument from message conversion

Change-Id: Ic727b7f61e7b2e38a36f5c2757271668ec143d77
Merge "Remove source as a publisher argument"
Merge "rpc: remove source argument from message conversion"