Adding toJson JNI method for Policy object

[mark-creamer-amazon]

#171

Backporting changes from #172

Tested these changes locally with the tip of cedar-java-hello-world 3.2.x

// permit(principal == User::"Alice", action == Action::"View_Photo", resource in Album::"Vacation");
Set<Policy> policySet = buildPolicySlice();
Policy policy = policySet.iterator().next();
System.out.println(policy.toJson());

{"effect":"permit","principal":{"op":"==","entity":{"type":"User","id":"Alice"}},"action":{"op":"==","entity":{"type":"Action","id":"View_Photo"}},"resource":{"op":"in","entity":{"type":"Album","id":"Vacation"}},"conditions":[]}