Fix storage API and databroker interaction on start up

Fix storage API and databroker interaction on start up

Changes:
- The persisted values read in by the forced early init of the
  storage API were getting overridden by the defaults coming from
  the databroker in the case of preferences that map directly to
  VSS signals. Refactor the preference updating code to move the
  VSS updating calls to reusable functions in ValClient, and then
  use those to update the VSS signal values right after connecting
  to the databroker.
- Remove the recursive initialization call from UnitsNotifier's
  loadSettingsUnits function, as in my testing it broke start up,
  and the recursion seems incorrect.  There are definitely issues
  with racing with the storage API daemon, and the homescreen
  completely hangs if it is not running.  The fix for that is to
  switch to a more Flutter typical asynchronous initialization of
  the storage API connection, and future rework will be in that
  direction.
- Change various prints in the storage API code to debugPrint to
  start trying to clean up complaints from "flutter analyze".

Bug-AGL: SPEC-5250

Change-Id: I6dc9a45569453254d1565038048305f94d121db6
Signed-off-by: Scott Murray <scott.murray@konsulko.com>