Fix samples on Android

Samples could not be built because ARFaceARKitBlendShapeVisualizer.cs references ARKit specific types which are not available on Android.

Assets/Scripts/ARFaceArkitBlendShapeVisualizer.cs

 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.Experimental.XR;
-using UnityEngine.Serialization;
+#if UNITY_IOS
+#endif
 
 /// <summary>
 /// Populates the action unit coefficients for an <see cref="ARFace"/>.
         }
     }
 
+#if UNITY_IOS
-
-    ARFace m_Face;
+#endif
+
+    ARFace m_Face;
+#if UNITY_IOS
+#endif
         m_Face = GetComponent<ARFace>();
         CreateFeatureBlendMapping();
     }
         {
             return;
         }
- 
+
+#if UNITY_IOS
         const string strPrefix = "blendShape2.";
         m_FaceArkitBlendShapeIndexMap = new Dictionary<XRArkitBlendShapeLocation, int>();
         
         m_FaceArkitBlendShapeIndexMap[XRArkitBlendShapeLocation.NoseSneerLeft       ]   = skinnedMeshRenderer.sharedMesh.GetBlendShapeIndex(strPrefix + "noseSneer_L");
         m_FaceArkitBlendShapeIndexMap[XRArkitBlendShapeLocation.NoseSneerRight      ]   = skinnedMeshRenderer.sharedMesh.GetBlendShapeIndex(strPrefix + "noseSneer_R");
         m_FaceArkitBlendShapeIndexMap[XRArkitBlendShapeLocation.TongueOut           ]   = skinnedMeshRenderer.sharedMesh.GetBlendShapeIndex(strPrefix + "tongueOut");
-        
+#endif
     }
 
     void SetVisible(bool visible)
 
     void OnEnable()
     {
+#if UNITY_IOS
-        if (m_ArkitFaceSubsystem == null )
-        {
+        if (m_ArkitFaceSubsystem == null)
-        }
+
+#endif
     }
 
     void OnDisable()
             return;
         }
 
+#if UNITY_IOS
-        {
-        }
 
         foreach (var xrFaceFeatureCoefficient in s_FaceArkitBlendShapeCoefficients)
         {
                 }
             }
         }
+#endif
     }
 }