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 "flutter-" prefix has been dropped from the configuration
  file name (i.e. it's now just "cluster-dashboard.yaml") to match
  the naming used for the other Flutter applications.  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 the other applications.
- The unused navigation support has been removed to simplify
  maintenance, as it is more likely that it will be replaced with
  something else in the future than fixed to be usable.
- 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.  Since pubspec.lock is
  not checked in here, it has also been added to .gitignore.

Bug-AGL: SPEC-4762

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: Id35c569cdbb8476a527717ece7b4bb369c4874b7

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