Re-enable dreams: Settings

Re-arrange settings and captions for dreams.  Activation on sleep
and activation on dock now independent. Sleep activation defaults to off,
dock activation defaults to on (replacing dock mode).

Also use dream manager service for testing dream preference, and properly
disable preferences if both dream activations are disabled.

Change-Id: I76f5ba115c7678826a09077dd393acc55c2bca26
Bug: 6921930