Move variations prefs into the variations component

These prefs are used by variations code that is targeted for componentization.

BUG=382865
TBR=thakis

Review URL: https://codereview.chromium.org/1265423003

Cr-Commit-Position: refs/heads/master@{#343661}
std::string VariationsSeedStore::GetInvalidSignature() const {
  return invalid_base64_signature_;
}

std::string VariationsSeedStore::GetInvalidSignature() const {
  return invalid_base64_signature_;
}
