Active Directory Import is new synchronization method introduced in SharePoint 2013. It is highly efficient if only import is required even with AD only.
And that's true, you will not be able to start User Profile Synchronization Service after configuration synchronization method as Active Directory Import
You have to perform some step to create connection with a account that have Replicate Directory Change Permission on a Domain
1.On the domain controller, click Start, click Administrative Tools, and then click Active Directory Users and Computers.
2.In Active Directory Users and Computers, right-click the domain, and then click Delegate Control.
3.On the first page of the Delegation of Control Wizard, click Next.
4.On the Users or Groups page, click Add.
5.Type the name of the synchronization account, and then click OK.
7.On the Tasks to Delegate page, select Create a custom task to delegate, and then click Next.
8.On the Active Directory Object Type page, select This folder, existing objects in this folder, and creation of new objects in this
folder, and then click Next.
9.On the Permissions page, in the Permissions box, select Replicating Directory Changes (select Replicate Directory Changes on Windows
Server 2003), and then click Next.
After performing this operation, you have to create a connection To Active Directory Import which you already did, so edit that file and select the container and click Ok there.
Then you have to map your AD property to User Profile Property. Do remember you to write an Attribute instead of selecting it from dropdown. As dropdown will be disabled for Active Directory Import synchronization method.
In my case, I map mail to Work Email in user profile as shown in below screenshot.
If mapping already exist but attribute is null there then remove that mapping by clicking Remove button. Then add an attribute there and click ADD. Finally click ok.
Note: Don't forget to click add button after adding an attribute there.
Then run the full synchronization, check whatever property you had mapped are successfully imported to User profile.
But still, User Profile Synchronization service is not running. It all work without synchronization service running in case of Active Directory Import synchronization method. And even you will not b e start to start it.
That's I don't know the exact reason behind it!
But finally all things work perfectly after doing steps that I mentioned above.