Put rollbacks into #mRollbacks (9/n)

Now we check Rollback#isNewRollback for rollbacks that were originally
in #mNewRollbacks.

for (Rollback newRollback : mNewRollbacks) {
  // Do something with newRollback...
}

will be replaced by:

for (Rollback newRollback : mRollbacks) {
  if (newRollback.isNewRollback()) {
    // Do something with newRollback...
  }
}

Since mRollbacks includes new rollbacks, be careful not to apply
operations not appropriate to new rollbacks when iterating over
mRollbacks. Luckily most of the code is future-proof that needs no
changes.

Note now #mNewRollbacks is always empty. We will remove it in the next
CL.

Bug: 147400979
Test: atest RollbackTest StagedRollbackTest
Change-Id: Ia3a4116b352228adc0b152d42c85920f375beb28