得到最顶端的activivty

Context context = someArbitraryContext;?
ActivityManager am = (ActivityManager) context.?? ? getSystemService(Activity.ACTIVITY_SERVICE);?
String packageName = am.getRunningTasks(1).get(0).topActivity.getPackageName();?
String className = am.getRunningTasks(1).get(0).topActivity.getClassName();?