The American economy is now strong enough to withstand Middle East turmoil and the Japanese nuclear crisis. Only a big rise in the price of oil could stop it now. Those are the findings of an Associated Press survey of leading economists, who are increasingly confident in a recovery that is nearly two years old. They expect the economy to grow faster every quarter this year. In part, that's because the economists think Americans will spend more freely in the coming months. Higher stock prices have made people wealthier. And a cut in the Social Security payroll tax is giving most households an extra $1,000 to $2,000 this year.