Offer the user an option to launch Dreams when docked.

SystemUI now registers for DESK_DOCK launches, so users with
other dock apps installed can still opt to use those in this
new regime.

(Part of migrating users away from DeskClock as the dock app.)

Bug: 3155234
Change-Id: I0da0f04f8a0a89e7d237c092f16f4f27eb88c92c