Have RollbackData reuse RollbackInfo.

Rather than duplicating the same information.

This is in preparation for storing available and recently committed
rollbacks the same way so we don't end up with duplicate copies of
PackageRollbackInfo for a rollback so we can fix the bug when doing
userdata restore for staged installs.

Bug: 124044231
Test: atest RollbackTest
Test: atest StagedRollbackTest
Test: atest AppDataRollbackHelperTest
Change-Id: I6ca164adc4351b778d153d4b33296386f6833b61