Show desk dock apps as screen savers.

Now that docking launches the screen saver instead of a dock
app, we offer those apps as screen savers to allow them to
continue functioning as they did before. As a bonus, users
without docks can now use dock apps!

Bug: 5591015
Change-Id: Ifc138ab4c0bdffa1bbd7afe8b3ad730ec971aa34