Short answer:
The URI passed to MediaHttpUploader's resumable upload when you use the Google Drive java client API seems to be the same as the one proposed on Google Drive API V2. So actually by default the Google Drive Java client API is already using the resumable upload.
Long answer:
After tracing the code in Google Drive Java API client all the way from
com.google.api.services.drive.Drive
->com.google.api.services.drive.DriveRequest
->com.google.api.client.googleapis.services.AbstractGoogleJsonClientRequest
You will find that Drive's constructor passes the URI to DriveRequest, which also passes the variable uriTemplate to AbstractGoogleJsonClientRequest. And finally AbstractGoogleJsonClientRequest uses buildHttpRequestUrl() to generate the resumable URI proposed in Google Drive API reference. This URI is stored in the variable httpRequestUrl in AbstractGoogleJsonClientRequest. httpRequestUrl will then be passed to uploader's upload method. This method by default (directUploadEnabled defaults to false) will use the resumable upload instead of direct upload.