packagekit: Fix progress marshalling for upgrade_download and refresh

The new GsPackageKitHelper class introduced in mega-commit 37103e6e only handled progress marshalling when downloading a list of packages, but not when the PK transaction is one big monolithic operation that only has a single GsApp associated, such as when downloading all of the distro upgrades, or when refreshing the cache.

This commit adds a way to explicitly add a GsApp to GsPackageKitHelper to use for marshalling progress in these cases.

Fixes: #620 (closed)