update NDK doxygen templates to create DevSite-style HTML.

Previously, we had doxygen create .jd files with some syntax that
was parsed by the DocLava build tools when creating the final HTML
files for the website. But now that we're publishing these docs through
Piper, we don't need to run them through DocLava anymore. So with this
change, we now create .html files that then need to be copied to
google3/googledata/devsite/site-android/en/ndk/
where they can be immediately published via devsite once merged.
bug: 29943712

Change-Id: I8af5555f802e8c0122378bd504ad4308ff9aad56