Remove a NewRollback immediately when cancelled (1/n)

This change allows us to remove #isCancelled and #setCancelled. By
reducing the difference b/t NewRollback and Rollback, it will be easier
to merge them later.

Bug: 147400979
Test: atest RollbackTest
Change-Id: I9bf626f252e828994b435f96ec429265fcec4ece