"Air conditioning" is closely related (one might even say integral) to the
cabin pressurization system. The fact they descended to 3000m (10000 feet) also indicates they had lost control of cabin pressure.
The emergency descent means 1) they were abandoning the filed flight plan and passing through altitudes possibly occupied by other aircraft, and 2) would end up at an altitude where their fuel burn rate would increase significantly, perhaps requiring a diversion (no longer enough fuel to reach Narita).
Fortunately they were relatively close to destination, so once they reached 3000m where pressurization was not necessary to support life, the crew determined they could cancel the alert and continue safely to Narita on the remaining fuel.
A
dual air-conditioning/pressurization "pack" failure is an emergency worthy of investigation. Both systems should not fail at the same time (that is why there are two). If it had happened a thousand miles further out over the Pacific, it could have ended up in a mid-ocean ditching.
See report of similar incident, coincidentally released June 4:
Incident: Vueling A320 near Alicante on Feb 21st 2017, both air conditioning systems failed