\ifxetex
detects whether you are compiling with XeTeX or not. If you are, it executes the if
bit. If not, it executes the else
bit. Moreover, you cannot use inputenc
with XeTeX (so it is good, really, that that code is never read).
You can use 'normal LaTeX fonts' for the rest of the document, though. Just use the three lines of code from \usepackage{fontspec}
and put \usepackage{droid}
configuration afterwards, as loading fontspec
sets the default families to Latin Modern.
\documentclass{article}
\usepackage{fontspec}
\defaultfontfeatures{Ligatures=TeX} % To support LaTeX quoting style
\newfontfamily\headerfont{Quintessential}
\usepackage[T1]{fontenc}
\usepackage{droid}
\begin{document}
Some text.
\end{document}
However, you may prefer to use the Droid fonts in opentype/truetype format to take full advantage of XeTeX.
\documentclass{article}
\usepackage{fontspec}
\defaultfontfeatures{Ligatures=TeX} % To support LaTeX quoting style
\newfontfamily\headerfont{Quintessential}
\setmainfont{Droid Serif}
\setsansfont{Droid Sans}
\setmonofont{Droid Sans Mono}
\begin{document}
Some text. Serif. \textsf{Sans}. \texttt{Mono}.
\end{document}

Note that I changed the value of \headerfont
to prevent XeLaTeX hanging forever since I don't have the font you are using.