How can I configure the Email app not to save sent messages on the Google server using IMAP?
I think there is no need to set those settings in the Email app. You only need to set them in e-mail clients where you are getting duplicate copies of the email, provided those clients have a way of setting those settings. 

If you notice in the image below (from the desktop GMail web app), there is a Sent Mail and an [Imap]/Sent folder. The Sent Mail folder is GMail's, while the [Imap]/Sent folder is generated by a 3rd-party client (like Thunderbird or Kaiten, for example). 



Ideally, a sent mail would be placed under Sent Mail or under [Imap]/Sent. In some cases, it gets copied under both. So now, you have two copies of the sent email. If this is happening, then you need to follow the Recommended IMAP client settings. If not, you're fine.

Related reference:
https://webapps.stackexchange.com/questions/16868/gmail-and-imap-how-do-the-imap-drafts-imap-sent-and-imap-trash-folders