DADAAB, Kenya — The Kenyan government is threatening to dismantle the world’s largest refugee camp, setting off a panic among the nearly 350,000 people who live here and the international aid organizations that care for them. Kenyan officials say the camp is a national security threat, a constellation of tents and huts used by the Somali extremist group al-Shabab to plan attacks, like the one on Garissa University College that killed 148 people last month.