Followup cleanup after refactoring rollback states.

This CL addresses some minor comments as followup to the previous CL
"Use a single list for available and committed rollbacks".

Test: atest RollbackTest
Test: atest StagedRollbackTest
Test: atest AppDataRollbackHelperTest
Bug: 124044231
Change-Id: I6146cfac3a70017dc8d7eed6b6d3c19de932f14b