ISLAMABAD, Pakistan — Prime Minister Imran Khan of Pakistan is fighting for his political survival after opposition political parties have moved for a no-confidence motion in Parliament and the country’s powerful military has withdrawn its support for his government. Mr. Khan, the former cricket-star-turned-politician, has announced plans to gather a million supporters in Islamabad, appealed to the Supreme Court to disqualify lawmakers who have defected from his party and denounced his critics as part of an American-influenced conspiracy. But as demands for his resignation mount, critics and analysts say he…