@padmalcom
What you are trying to do here isn't possible I'm afraid; not in the Maps SDK alone.
The offline map tasks which generates a mobile map package is for use with a web map which is published in ArcGIS Online or your own Enterprise environment. The creation of the mobile map package is a server side operation and there is no logic in the Maps SDK to perform this operation.
The solution here might be to publish your vector tile data into a webmap and you can use that to request a smaller area using the offline workflows.
I should also make you aware that the Maps SDK for Java is being deprecated this year so you may want to consider using one of the other Native Maps SDKs for a new project.
Does that help?