Use PackageRollbackInfo#getVersionRolledBackFrom to simplify code (2/n)

PackageRollbackInfo#getVersionRolledBackFrom returns a VersionedPackage
which contains a version code. No need to query the version code from
the package manager.

Bug: 147028082
Test: atest StagedRollbackTest
Change-Id: I769e726183ee99bb9e8c7576bb8b6c21a75fe2ff
1 file changed