Minimize app dependency on platform service (#21548)

api4/post.go

@@ -103,7 +103,7 @@ func createPost(c *Context, w http.ResponseWriter, r *http.Request) {
 		c.App.SetStatusOnline(c.AppContext.Session().UserId, false)
 	}
 
-	c.App.UpdateLastActivityAtIfNeeded(*c.AppContext.Session())
+	c.App.Srv().Platform().UpdateLastActivityAtIfNeeded(*c.AppContext.Session())
 	c.ExtendSessionExpiryIfNeeded(w, r)
 
 	w.WriteHeader(http.StatusCreated)