Start the dream manager even if dreams are not supported.

The dreams manager also manages dozing.  It has a minimal footprint
so there is no real reason to disable the component (it just makes
debugging more difficult).

Improved the documentation of the config_dreamsSupported resource
to clarify exactly what it controls.

Bug: 12494706
Change-Id: I78244846f7c1ddfd11bc1605af59b0db91337971