B747 Captain reported a momentary stick shaker during approach resulting from the speed brake being improperly armed.

Narrative

On turning final at 15 miles at Runway XXL at ZZZZ; ATC asked us to reduce to 160 kts. VRef 30 was 155 kts; additive was 5 kts - Final approach speed was 160 kts. At flaps 25 (for a flaps 30 landing) aircraft was stable at 160 kts. First Officer (FO) (an OE student on second landing on type) was attempting to arm the speed brake for the first time in a real aircraft and pulled the lever past the Armed detent. Stick shaker activated momentarily (crew estimate less than a second). Captain was reaching for the controls to execute the stall recovery procedure however the FO had returned the speed brake to the down detent and; well before the Captain's hands were on the controls; the warning had ceased. Airspeed never decreased below VRef 30. The speed did not deteriorate below VRef 30; rather the Minimum Maneuvering Speed increased above VRef momentarily. At the same time this was happening; there was some significant localiser oscillation causing the aircraft to turn gently to the left and right.