Use initial_preferences to supply griffin seed for new installations

[iefremov]

Currently, griffin provides clients only with seed, fetched from Brave endpoints. Given that the browser needs to restart to apply a new seed, new Brave installations don't have the actual griffin state on a first launch, since they never restarted.

To fix that we'd need to supply initial_preferences containing the initial seed to fresh installations.

initial_preferences is a JSON that should be located in the same directory as the brave binary executable. Each brave installer (win/mac/lin/android) should contain this file. The file should have the following format:

{
  "variations_compressed_seed": "<base64-encoded gzipped protobuf same as on https://variations.brave.com/seed>,
  "variations_seed_signature": "<seed signature returned in x-seed-signature from the variations endpoint"
}

[goodov]

Windows: https://github.com/brave/devops/issues/10466
Android: #33141