The basic text editor is built into Android, and it can display text with certain types of formatting, but has no UI controls for creating / changing the styling.
What I did is start with an actual editor (all the stuff that lets the user actually apply formatting), written by Mark Murphy, who is a world-famous Android author / trainer / expert... he had the source code up on GitHub under an acceptable license.
... and then I did a fairly large amount of work to make it work better (e.g. being able to press "Bold" when there is no selection, then when you start typing, the new text will actually be bold...), compatibility fixes for Android 2.*... the floating icon bar...
Then the code to save and restore the formatting, converting to HTML for sending, oh, it really was a lot of work to make it usable. I'm very grateful to Mr. Murphy for his work, though.
The final step was adding support for embedded images, this is heavily dependent on the rest of the app -- since those images need to be stored somewhere, loaded from storage as needed (in the background, so the UI doesn't freeze), cleaned up when the user deletes an image... The popup menu for resizing and attaching a link, all of that...
