The android platform is not interested in this feature of Mesa.  There are
currently workarounds for apps on Android, and no support for it in the
xmlconfig code.  Even if there we do need workarounds eventually, we'll
want to bake them in as structs rather than have this awkward external
dependency for parsing user-readable data installed by Mesa for
Mesa-internal details.

This gets rid of the expat dependency in the turnip driver.

Note that rather than have more #ifdefs in the file, I've opted to move
the code to have more logical locations since the structs refactor had
left less-used helpers scattered across the file.

Acked-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6916>