Add multi-package support to RollbackData.

In preparation for multi-package rollback, update the RollbackData data
structure to support multiple packages in a single rollback.

Does not yet add support for enabling rollback of multi-package
installs.

Test: atest RollbackTest
Bug: 112431924
Change-Id: Id7e8efaaa829f953dabaeb9ec8959884c027dffe