pk: Don't call the size function as it triggers a details fetch

We are only checking the previous size to see if it needs to emit a
change, we were fetching twice in practice.

CCBUG: 432657

libdiscover/backends/PackageKitBackend/PackageKitResource.cpp

@@ -198,7 +198,7 @@ void PackageKitResource::setDetails(const PackageKit::Details & details)
 
     if (m_details != details) {
         const auto oldState = state();
-        const auto oldSize= size();
+        const auto oldSize = m_details.size();
         m_details = details;
 
         if (oldState != state())
@@ -207,7 +207,7 @@ void PackageKitResource::setDetails(const PackageKit::Details & details)
         if (!backend()->isFetching())
             Q_EMIT backend()->resourcesChanged(this, {"size", "homepage", "license"});
 
-        if (oldSize != size()) {
+        if (oldSize != uint(size())) {
             Q_EMIT sizeChanged();
         }
     }