UI: change default avatar

Change-Id: Ib40c7ec9f6ae434e48cd8ec8dcd6826299daa581
Reviewed-by: Kateryna Kostiuk <kateryna.kostiuk@savoirfairelinux.com>

Ring/Ring/Resources/Images.xcassets/ic_contact_picture.imageset/Contents.json

@@ -2,11 +2,12 @@
   "images" : [
     {
       "idiom" : "universal",
-      "filename" : "ic_contact_picture.png",
+      "filename" : "ic_contact_avatar.png",
       "scale" : "1x"
     },
     {
       "idiom" : "universal",
+      "filename" : "ic_contact_avatar@2.png",
       "scale" : "2x"
     },
     {