Fire "dreaming started" and "dreaming stopped" broadcasts.

Dream manager now fires broadcast intents when entering + exiting
dreamland (except when testing).

Power manager can now listen for dreams ending, using polling only
as a backstop.

Also:
 - Bullet-proof dream-manager/dream against known failure modes
 - Add new read/write dream permissions
 - Refactor dream-manager to delegate work + state management into
   a new DreamController class, via a handler

Bug:6999949
Bug:7152024
Change-Id: I986bb7812209d8c95ae1d660a5eee5998a7b08b1