Refactor DoH implementation

Current doh implementation is designed to be totally managed by
DnsResolver, which means we might need bunch of C++ glue code to make
things work.
This goal of this refactoring is to minimize the required C++ glue code
and put most control logic into Rust side.

Test: atest
Bug: 155855709
Change-Id: I9a048f5fe72c4c25ae1b95ddf839f244eda34097