UI: Use better icon for identifying assets

Use the `ASSET_MANAGER` icon which is more appropriate than the current one
which was just an unused icon that seemed sorta fitting, but was only meant to
be temporary.
The `ASSET_MANAGER` icon is already used for the Asset Browser, so it's being
reused which we normally avoid. So we may still want to create a dedicated one,
maybe a variation of this one.

source/blender/editors/interface/interface_icons.c

@@ -2200,7 +2200,7 @@ int UI_icon_from_library(const ID *id)
     return ICON_LIBRARY_DATA_OVERRIDE;
   }
   if (ID_IS_ASSET(id)) {
-    return ICON_MAT_SPHERE_SKY;
+    return ICON_ASSET_MANAGER;
   }
 
   return ICON_NONE;