How to get Owner of a Build Definition from RTC side via RTC API

As far as I  know there s no Owner for an com.ibm.team.build.common.model.IBuildDefinition
There is a project or Team Area property defined by the property PROPERTY_PROCESS_AREA.

From the IBuildDefinition it is possible to get the IProcessAreaHandle directly, which is a superinterface of both IProjectAreaHandle and ITeamAreaHandle.

The example below is complete and will print the unique UUID of the Process Area. The same you would see in the Project/Team Area URI.

public class Main {
  public static void main(String[] args) {

    IProgressMonitor myProgressMonitor = new NullProgressMonitor();
    final String userId = "abc0de";
    final String password = "********";
    String repoUri = "https://server.com/ccm/";
    TeamPlatform.startup();
    try {
      ITeamRepository repo = TeamPlatform.getTeamRepositoryService().getTeamRepository(repoUri);
      repo.registerLoginHandler(new ILoginHandler2() {
        @Override
        public ILoginInfo2 challenge(ITeamRepository repo) {
          return new UsernameAndPasswordLoginInfo(userId, password);
        }
      });
      repo.login(myProgressMonitor);

      /* Use the repo to get the build client library */
      ITeamBuildClient buildClient = (ITeamBuildClient) repo.getClientLibrary(ITeamBuildClient.class);
      /* Get the build definition for "My Build Definition" */
      IBuildDefinition definition = buildClient.getBuildDefinition("My Build Definition", myProgressMonitor);
      IProcessAreaHandle processAreaHandle = definition.getProcessArea();
      System.out.println(processAreaHandle.getItemId());
      repo.logout();
    }
    catch (TeamRepositoryException e) {e.printStackTrace();}
    finally {
      TeamPlatform.shutdown();
    }
  }
} 

Hope it helps!

Edit:
As Ralph Schoon mentioned, you cannot assign a USER as owner of a Build Definition. Instead, you can set an area (Project Area or Team Area) where it's visible and its users are allowed to modify/delete it based on roles and privileges assigned.
In RTC, some elements can be assigned to users and project/team areas (i.e. Components), only users (i.e. Repository Workspace) or only project/team areas (i.e. Streams).