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
1 job for wip/kalev/fix-upgrade-download-progress in 22 minutes and 1 second (queued for 1 second)
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passed #293030
build-gnome-software

00:22:01