Fixup std::variant usage

This change refactors mapbox::variant specific usage onto an API that
mapbox::variant and std::variant both support.

Tested:
    Run through unit tests.

Change-Id: Ic35953b4772398495129830ed0d4455409f844ea
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/manager.cpp b/manager.cpp
index 5e05342..2342743 100644
--- a/manager.cpp
+++ b/manager.cpp
@@ -194,7 +194,9 @@
 
     const auto& properties = associations->second;
     auto assocProperty = properties.find("associations");
-    auto assocValue = assocProperty->second.get<AssociationsPropertyType>();
+    auto assocValue =
+        sdbusplus::message::variant_ns::get<AssociationsPropertyType>(
+            assocProperty->second);
 
     auto id = getEntryID(objectPath);
     auto calloutNum = 0;
@@ -317,7 +319,8 @@
         auto property = interface->second.find("Timestamp");
         if (property != interface->second.end())
         {
-            return property->second.get<uint64_t>();
+            return sdbusplus::message::variant_ns::get<uint64_t>(
+                property->second);
         }
     }