Make DreamManagerService more robust.

Clearly isolated the DreamManagerService and DreamController
responsibilities.  DreamManagerService contains just enough logic to
manage the global synchronous behaviors.  All of the asynchronous
behaviors are in DreamController.

Added a new PowerManager function called nap() to request the device
to start napping.  If it is a good time to nap, then the
PowerManagerService will call startDream() on the DreamManagerService
to start dreaming.

Fixed a possible multi-user issue by explicitly tracking for
which user a dream service is being started and stopping dreams
when the current user changes.  The user id is also passed to
bindService() to ensure that the dream has the right environment.

Fix interactions with docks and the UI mode manager.  It is
important that we always send the ACTION_DOCK_EVENT broadcast
to the system so that it can configure audio routing and the like.
When docked, the UI mode manager starts a dock app if there is
one, otherwise it starts a dream.

This change resolves issues with dreams started for reasons other
than a user activity timeout.

Bug: 7204211
Change-Id: I3193cc8190982c0836319176fa2e9c4dcad9c01f