Add a flag to facilitate merging 2 rollback collections (8/n)

We will move rollbacks from #mNewRollbacks to #mRollbacks in the next
CL. To preserve the same semantics for new rollbacks, rollbacks
originally managed by #mNewRollbacks will have
Rollback#mIsNewRollback==true.

Bug: 147400979
Test: atest RollbackTest
Change-Id: I7edd3cf75d40ff2a61c485efc0cc4ec7de7d4f99