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

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