modules: camera: make Camera object id const

Change-Id: Ic36fa7c6193086aa6415647ff8fe59cc358dd99f
diff --git a/modules/camera/Camera.h b/modules/camera/Camera.h
index f38b461..f2ad093 100644
--- a/modules/camera/Camera.h
+++ b/modules/camera/Camera.h
@@ -51,7 +51,7 @@
 
     private:
         // Identifier used by framework to distinguish cameras
-        int mId;
+        const int mId;
         // Busy flag indicates camera is in use
         bool mBusy;
         // Camera device operations handle shared by all devices