Continue to next config level after device for features

This ensures we continue checking further config levels for the feature setting
if nothing is found when reading the device level. For example, this means the
feature setting's default value can be used.

src/settings/handlers/DeviceSettingsHandler.js

@@ -122,7 +122,10 @@ export default class DeviceSettingsHandler extends SettingsHandler {
         }
 
         const value = localStorage.getItem("mx_labs_feature_" + featureName);
-        return value === "true";
+        if (value === "true") return true;
+        if (value === "false") return false;
+        // Try to read the next config level for the feature.
+        return null;
     }
 
     _writeFeature(featureName, enabled) {