Add "exclusive" attribute for roles.

This changes adds the "exclusive" attribute for roles, which is
required for role definitions in XML. If this attribute is set to
true, role controller will make sure there is at most one holder for
the role.

Test: build
Bug: 110557011
Change-Id: I119cf353ffdaee2f49e56e5a64b9ebc0dc0f9a9b