Simplify AsyncDnsResolver callback lifetime management

In local testing this reduces ResolveAfterDeleteDoesNotReturn flake.
https://ci.chromium.org/ui/p/webrtc/builders/luci.webrtc.ci/Win64%20ASan?limit=200

There's one more thing we can do to fully address this, but I'll
do that in a separate cl since it involves more windows specifics.

Example failure:
https://ci.chromium.org/ui/p/webrtc/builders/ci/Win64%20ASan/17188/overview

* Update AsyncDnsResolver::State to directly guard the target TaskQueue.
* Change the callback type to `absl::AnyInvocable<void() &&>` to
  clarify that it is called at most once.
* Simplify tests and remove unused dependencies.
* Instead use RunLoop, which was already there, instead of the
  polling mechanism.
* Remove the use of Sleep().

Bug: none
Change-Id: If8d1455a2f580d01b0245ca0c8f96013613994e0
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/423320
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Tomas Gunnarsson <tommi@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46351}