Fix memory leak in resolver cache

Memory leaks in resolver cache when network is being destroyed.
We clear DNS servers before deleting the cache to make sure the
cache will not be created again.

Bug: 122940321
Test: built, flashed, booted
      system/netd/tests/runtests.sh passed

Change-Id: I6aacc13ade3ce020b906f8a8a0f870f0741a5a47