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}
void VariationsRequestSchedulerMobile::OnAppEnterForeground() {
  if (base::Time::Now() <
      last_request_time_ + base::TimeDelta::FromHours(kSeedFetchPeriodHours)) {
    return;
  }

  schedule_fetch_timer_.Start(
      FROM_HERE,
      base::TimeDelta::FromSeconds(kScheduleFetchDelaySeconds),
      this,
      &VariationsRequestSchedulerMobile::Start);
}

void VariationsRequestSchedulerMobile::OnAppEnterForeground() {
  if (base::Time::Now() <
      last_request_time_ + base::TimeDelta::FromHours(kSeedFetchPeriodHours)) {
    return;
  }

  schedule_fetch_timer_.Start(
      FROM_HERE,
      base::TimeDelta::FromSeconds(kScheduleFetchDelaySeconds),
      this,
      &VariationsRequestSchedulerMobile::Start);
}
