It seems, that packagekitʼs APT backend do not respect APT default release (aka target release).

## Steps to reproduce

/etc/apt/sources.list

	deb https://deb.debian.org/debian/ sid main
	deb https://deb.debian.org/debian/ experimental main

/etc/apt/apt.conf.d/00default-release

	APT::Default-Release "sid";

/etc/apt/preferences.d/20experimental-is-okay

	Package: *
	Pin: release a=experimental
	Pin-Priority: 500

(would not be required for testing and unstable on stable system).

So that:

	$ apt-cache policy
	Package files:
	 100 /var/lib/dpkg/status
	     release a=now
	 500 https://deb.debian.org/debian experimental/main amd64 Packages
	     release o=Debian,a=experimental,n=experimental,l=Debian,c=main,b=amd64
	     origin deb.debian.org
	 990 https://deb.debian.org/debian sid/main amd64 Packages
	     release o=Debian,a=unstable,n=sid,l=Debian,c=main,b=amd64
	     origin deb.debian.org
	Pinned packages:

Compare

	$ apt list --upgradeable

(no extra upgrades should be found) with

	$ pkcon get-updates

which suggests to upgrade everything possible to experimental.

## Expected result

packagekit-based software (pkcon, plasma-discover, apper, etc) behaves mostly the same way as apt(8), aptitude(8), libqapt3-based software (e. g. muon) does.

## Workaround

Emulate default release with apt_preferences(5):

/etc/apt/preferences.d/00emulate-default-release

	Package: *
	Pin: release n=sid
	Pin-Priority: 990