Add a hidden API for detecting response charsets.

This is meant to be a partial replacement for apache's
EntityUtils.toString for internal callers. Providing a stand
in replacement seems like a bad idea because that API tries
to coerce all responses into strings, even those that are
binary.

I'm torn about EntityUtils. It was a useful "utility" in some
cases, but dangerous in others.

bug: 18027885
Change-Id: Ia02ffe86b5ef5caecbe86069967a549ff09335e5