Apple chief executive Tim Cook will hope that the launch of the latest iPhones on Tuesday will be enough to buoy consumers and investors in the face of a falling share price caused by deteriorating international relations. Almost $200m (£160m) was wiped from this most valuable tech stock last week as escalating tensions between Beijing and Washington threatened to restrict sales in one of its biggest markets: China accounts for roughly a fifth of Apple’s revenue. Reports of the Chinese government clamping down on use of the American devices by state employees may be having a wider chilling effect as…