@DavidSonPM was also able to weigh in on your question here during his 7.21.21 Adobe Target Community Q&A Coffee Break! His answer below:
" Yes, there are a couple of ways you can turn on / off on-device decisioning for the Java SDK
1. When you initialize the Java SDK you can set the default decisioningMethod variable for all decisions. You can set it to ON_DEVICE or HYBRID. Setting it to ON_DEVICE will have all requests execute on-device while HYBRID will ask the SDK to make the decision for you whether it may be server-side or on-device depending on the activity that is created.
2. You can override at the getOffers() method to choose which decisioningMethod you want to use request by request."