Allocate resolv_cache_info with operator new

Since there are more and more C++ containers in resolv_cache_info,
it's not safe the code keeps using malloc/free for the struct because
the destructors don't run when free() is called.

Bug: 130686826
Test: cd packages/modules/DnsResolver && atest
Change-Id: Ia4bc47a066b0814d62c7dfaf53325ab9ce890332