Rework to use KUKSA.val databroker gRPC API

Rework to use KUKSA.val databroker gRPC API

Rework to move from the WebSocket API with the older KUKSA.val
server to the gRPC "VAL" API of the databroker.

Changes include:
- All VISS WebSocket API code has been removed, and the signal
  providers replumbed to be driven by a new VssClient class with
  a dashboard-specific child class to hold all the gRPC API
  handling.
- The generated code for the VAL API and its dependencies has
  been checked in under lib/generated, as there still does not
  seem to be a good way to generate it during the Flutter build.
- The configuration file is now expected to be "dashboard.yaml"
  instead of "dashboard.yaml".  The authorization token
  field name has been renamed to "authorization", and there are
  new "use-tls" and "ca-certificate" configuration fields.  TLS
  is disabled by default for now, and the default CA certificate
  is /etc/kuksa.val/CA.pem.
- Bumped minimum SDK version to 2.18 in pubspec.yaml to enable
  "super" keyword support.  This matches what the version was set
  to in flutter-homescreen.
- Removed .dart_tool generated output that had been checked in,
  and added .gitignore file from flutter-homescreen so that things
  will hopefully stay clean in the future.

Bug-AGL: SPEC-4762

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: Ie23091b759458b5a131bc2d85e62925238839166