fix(ios): don't call urlSchemeTask methods if it was stopped

[jcesarmobile]

keep track of urlSchemeTasks and don't call their methods if the task was stopped

closes #7404

[Steven0351]

Instead of using lock.withLock, we should probably use the individual locking methods so the code would look like this:

func withTask(_ schemeTask: WKURLSchemeTask, action: @escaping () -> Void) {
  lock.lock()
  if tasks.contains(schemeTask.hash) {
    DispatchQueue.main.async {
      action()
      lock.unlock()
    }
  } else {
    lock.unlock()
  }
}

In it's currently state, the lock is can actually be released before the action is run because DispatchQueue.main.async returns immediately and would still allow for the race condition to occur.