Tweak the lifecycle of rollbacks (2/n)

1. #createNewRollbackLocked now adds newly created rollbacks to
   mRollbacks
2. #completeEnableRollback no longer adds the rollback object to
   mRollbacks.
3. #completeEnableRollback delete and remove rollbacks from
   mRollbacks should any error happen.

Bug: 149258640
Test: atest RollbackTest StagedRollbackTest
Change-Id: Ie6668cf6bd00ce9b83096383776bc75198c4f194