TWebBrowser
Is a particular firemonkey Control. Firemonkey control are "paint" according to their z-order on a openGL surface (ie: the form). but TwebBrowser is a native control that is draw on another surface that is placed on the top of the form, hidding in this way everything back to it.
The only way you can do is to show you memo inside another window on the top of the TwebBrowser. for this you can use a native Memo. As far as i know their is only one native memo implemented on android (delphi already have some that work on ios/windows) it's https://github.com/Zeus64/alcinoe