Hook up ARKitSettings in Build Settings

An ARKitSettings existed, but was not actually used by the build
settings.

ProjectSettings/EditorBuildSettings.asset

   - enabled: 0
     path: Assets/Scenes/FeatheredPlaneScene.unity
     guid: 8252358a1e2ba7549a94df43aacec984
-  m_configObjects: {}
+  m_configObjects:
+    com.unity.xr.arkit.PlayerSettings: {fileID: 11400000, guid: 2d5fb8e61571b4e3897f221189089fd6,
+      type: 2}