VSS signal fixes

VSS signal fixes

Changes:
- Fix sets of HVAC and volume control signals to update the
  actuator target value instead of the current value.  With the
  KUKSA.val ecosystem, it is the actuator implementation / backend
  that updates the current value after actuating.
- Add authorization token to subscribe request metadata if it is
  present.
- Switch type used for engine speed to double to match VSS signal
  definition.  This fixes receiving updates, and avoids needing
  to convert types as well.
- Added logic to push out the HVAC fan speed setting and handle
  external updates.
- Remove unused core/constants/paths.dart file to avoid confusion
  over VSS signals used and their variable naming.

Bug-AGL: SPEC-4999

Change-Id: Ifbf69af25e3c563e9c707a4145089e4242b89bcd
Signed-off-by: Scott Murray <scott.murray@konsulko.com>