Rewrite the handling of session finished with success (4/n)

We only want to handle non-staged rollbacks here. Staged rollbacks are
handled elsewhere. Checks for new rollbacks can be replaced by
|!rollback.isStaged() && rollback.isEnabling()|.

Bug: 149069841
Test: atest RollbackTest StagedRollbackTest
Change-Id: I9488f94a76fdd4cedbfc494f9378d0baa054e057