HtmlOutputRendererHelper: fall back to page language
HtmlOutputRendererHelper should not crash hard if the ParserOutput has
no language set. ParserOutput may come from a variety of places, we
should be lenient about it not having a language.
However, we should try harder to actually set a language on ParserOutput
if we have one available. So this also updates
PageBundleParserOutputConverter to keep the ParserOutput's language in
sync wit the language header in the PageBundle.
Bug: T349868
Bug: T353689
Bug: T359426
Change-Id: I2edf20dc3b199e22cda2f32bc858c21ca7d8f4bd