[libprocessgroup] Use new jsoncpp API

Reader / StyledWriter are deprecated upstream

Bug: 170642246
Change-Id: Iadc944af17cad9640ed31364116d16f2b971434f
diff --git a/libprocessgroup/task_profiles.cpp b/libprocessgroup/task_profiles.cpp
index 8d4ce25..f13a681 100644
--- a/libprocessgroup/task_profiles.cpp
+++ b/libprocessgroup/task_profiles.cpp
@@ -425,10 +425,12 @@
         return false;
     }
 
-    Json::Reader reader;
+    Json::CharReaderBuilder builder;
+    std::unique_ptr<Json::CharReader> reader(builder.newCharReader());
     Json::Value root;
-    if (!reader.parse(json_doc, root)) {
-        LOG(ERROR) << "Failed to parse task profiles: " << reader.getFormattedErrorMessages();
+    std::string errorMessage;
+    if (!reader->parse(&*json_doc.begin(), &*json_doc.end(), &root, &errorMessage)) {
+        LOG(ERROR) << "Failed to parse task profiles: " << errorMessage;
         return false;
     }