I can see two options here that depends on your needs:
1. use console server REST api if you plan to employ the usage of console in your case as well, not talking about using it from the smartphone app but rather for admins/operators to perform additional actions on the processes and tasks
2. if option one is not valid in your case then I would go for using jBPM API instead of adding unnecessary layer (console server REST api) that will do pretty much the same what you need to do while embedding jBPM into your application
HTH