Use VersionedPackage in PackageRollbackInfo.

Rather than defining a new custom PackageRollbackInfo.PackageVersion
type.

Also clean up PackageRollbackInfo API by replacing public fields with
methods and picking better names.

Bug: 112431924
Test: atest RollbackTest
Change-Id: I68a91e07b8745df9c5ecb22fdccbfcd76385814a
5 files changed