Update blank file and folder icons for ATV server browsing

Signed-off-by: Felix Paul Kühne <fkuehne@videolan.org>

Apple-TV/Assets.xcassets/NetworkBrowsing/blank.imageset/Contents.json

@@ -2,7 +2,7 @@
   "images" : [
     {
       "idiom" : "tv",
-      "filename" : "BlankFile.png",
+      "filename" : "doc-icon.png",
       "scale" : "1x"
     }
   ],

Apple-TV/Assets.xcassets/NetworkBrowsing/folder.imageset/Contents.json

@@ -2,7 +2,7 @@
   "images" : [
     {
       "idiom" : "tv",
-      "filename" : "Folder.png",
+      "filename" : "folder-icon.png",
       "scale" : "1x"
     }
   ],