EntityManager: Determine device config presence with .contains()

This is less verbose than .find() == .end()

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: I509463d5811415804dc2136aec5ea530318935b3
diff --git a/src/EntityManager.cpp b/src/EntityManager.cpp
index 78a3ca7..08797ac 100644
--- a/src/EntityManager.cpp
+++ b/src/EntityManager.cpp
@@ -884,8 +884,7 @@
             bool powerOff = !isPowerOn();
             for (const auto& item : lastJson.items())
             {
-                if (systemConfiguration.find(item.key()) ==
-                    systemConfiguration.end())
+                if (!systemConfiguration.contains(item.key()))
                 {
                     bool requirePowerOn = deviceRequiresPowerOn(item.value());
                     if (powerOff && requirePowerOn)