Refactor DnsResponderClient::GetResolverInfo

Refactor GetResolverInfo() to return a wrapper, so callers won't
have to define several variables to pass them to the function.

Apart from that, this change fixes a bug that the DnsResolver crashes
if wait_for_pending_req_timeout_count is an empty vector.

Bug: 237766689
Test: atest
Change-Id: I4ce05be760eca7f52e36c9b0b79852a054bfc757