update background image

Change-Id: I4bfdfcf37a732714e8646fcbbb007b7aa49af5a2

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

 {
   "images" : [
     {
-      "filename" : "Background _Light.heic",
+      "filename" : "bg_light.heic",
       "idiom" : "universal"
     },
     {
@@ -11,7 +11,7 @@
           "value" : "dark"
         }
       ],
-      "filename" : "Background _Dark.heic",
+      "filename" : "bg_dark.heic",
       "idiom" : "universal"
     }
   ],