Activate Dreams (screen saver) when desk-docked.

SystemUI's DreamsDockLauncher now responds to low-level dock
events (Intent.ACTION_DOCK_EVENT) by immediately invoking
the user's selected screen saver.

(The high-level DESK_DOCK behavior is no longer enabled; see
change I7884f65.)

Bug: 5591015
Change-Id: I01a9bc6b181d55493f891ccc220631ddc461808a