Factor out getPowerState() helper function

This same pattern had been open-coded in many of the sensor daemons;
let's reduce the duplication a bit.

Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Change-Id: I8556af30fdcd53cdf06e5a4911f9152d3265c7af
diff --git a/src/ADCSensorMain.cpp b/src/ADCSensorMain.cpp
index cec0984..fe8334d 100644
--- a/src/ADCSensorMain.cpp
+++ b/src/ADCSensorMain.cpp
@@ -253,14 +253,7 @@
                 }
             }
 
-            auto findPowerOn = baseConfiguration->second.find("PowerState");
-            PowerState readState = PowerState::always;
-            if (findPowerOn != baseConfiguration->second.end())
-            {
-                std::string powerState =
-                    std::visit(VariantToStringVisitor(), findPowerOn->second);
-                setReadState(powerState, readState);
-            }
+            PowerState readState = getPowerState(baseConfiguration->second);
 
             auto& sensor = sensors[sensorName];
             sensor = nullptr;