[ads] Educate users on the mobility and stickiness of custom notification ads

[tmancey]

@aguscruiz would be great if you could point us to the Figma designs. Thanks

[aguscruiz]

Sure, here you go!
https://www.figma.com/file/epdgFnOJIouJndgcejqYRE/Custom-Ads?type=design&node-id=52-10546&mode=design

[aseren]

We possibly need to educate user that the popup is a Brave push notification ad.
cc @iambrianfung @tmancey @aguscruiz