Rename PackageRollbackData RollbackData.

In preparation for supporting multi-package rollbacks, where
RollbackData will represent information for all packages in the atomic
set, rather than a particular package from the set.

Test: atest RollbackTest
Bug: 112431924
Change-Id: I9c94d2f5a6ea0ec9f0082cf9502e0ae517e83f99