Use ReaderFrontLight:addToMainMenu in FM

Check if frontlight is present using the same method as ReaderFrontLight:init,
just for consistency.

frontend/apps/filemanager/fmmenu.lua

@@ -49,13 +49,8 @@ function FileManagerMenu:setUpdateItemTable()
 		widget:addToMainMenu(self.tab_item_table)
 	end
 
-	if Device:hasFrontlight() then
-		table.insert(self.tab_item_table.main, {
-			text = _("Frontlight settings"),
-			callback = function()
-				ReaderFrontLight:onShowFlDialog()
-			end
-		})
+	if Device:getFrontlight() ~= nil then
+		ReaderFrontLight:addToMainMenu(self.tab_item_table)
 	end
 	table.insert(self.tab_item_table.main, {
 		text = _("Help"),