Keep 'partition' in PackagingSpec

Add a property 'partition' to PackagingSpec and fill it when
PackagingSpec is created.

This will enable PackageModule to filter PackgingSpec based on
partition. (Will be used in a follow-up change)

Bug: 225121718
Test: m (nothing changes)
Change-Id: I7657edd380e215df66bae57d8b07dd4181841fdf