If you include EXTRA_HTML_TEXT, that is supposed to be an alternative format to the plain text in EXTRA_TEXT. So maybe don't put HTML code in EXTRA_TEXT if you also put it in EXTRA_HTML_TEXT?
Note that not all (or very few?) email apps actually support EXTRA_HTML_TEXT, which is why EXTRA_TEXT is also required. So it could be that on your device, EXTRA_HTML_TEXT is being ignored and displaying EXTRA_TEXT as plain text instead, which might explain why you are seeing the HTML code as plain text instead of as formatted text.
Also, when adding HTML code to EXTRA_TEXT or EXTRA_HTML_TEXT, try passing it through the Html.FromHtml() method first, rather than adding it as a plain string.