I would like the ability to upload an APK, but not assign it to any track, basically leaving it as a draft. We prefer to manually assign it to a track and do a staged rollout.

According to the following code sample from Google, it appears that the upload and the track assignment are two separate commands.

https://github.com/googlesamples/android-play-publisher-api/blob/master/v2/java/src/com/google/play/developerapi/samples/BasicUploadApk.java#L74

https://developers.google.com/android-publisher/api-ref/edits/apks/upload

https://developers.google.com/android-publisher/api-ref/edits/tracks/update