By Andrew MacAskill KIGALI (Reuters) – British Prime Minister Boris Johnson said on Saturday he aims to remain in power until the middle of the next decade, despite calls for him to quit, which would make him the country’s longest continuously serving leader in 200 years. Earlier this month, Johnson survived a vote of confidence by Conservative lawmakers in which 41% of his parliamentary colleagues voted to oust him, and he is under investigation for intentionally misleading parliament. On Friday Conservative candidates lost two parliamentary by-elections held to replace former Conservative in…