False values are less than true values in Javascript (0 < 1); that's why employees with mobile = false
come first after you have them sorted by that property.
So in order to accomplish what you want, you just need to change the sort order in orderBy
by prefixing the property name with a - (dash):
$scope.employees = $filter('orderBy')($scope.employees, '-mobile');
Working plunk here.
You can find more information on the documentation about the orderBy
filter.