At the moment Android does not support signature on the screen. What I have seen in some implementations is that you have a written form and then define a Data Element of type Image (called consent, for example). When the pacient comes, they sign the document and you take a picture from the phone.
If you would like to have this signature embedded in the App I invite you to submite a JIRA feature request explaining with details your use case. https://dhis2.atlassian.net/