WASHINGTON — Sen. Kirsten Gillibrand is moving aggressively toward an expected presidential bid, filling out key staff positions and nearing a choice on the location for a campaign headquarters, according to multiple people familiar with the discussions. The New York Democrat is considering a February announcement, according to one person familiar with the discussions, who cautioned that the partial government shutdown could have an impact on the timing. Still, Gillibrand is filling out important staff positions, including recruiting Meredith Kelly, the former spokeswoman for the Democratic Congressional Campaign Committee, to head her communications efforts, according to the person.