Add resolv_has_nameserver() to replace getHasDns()

getHasDns() returns the value assigned from networkCreateVpn(). It causes
issues when DNS configuration is changed without triggering VPN reconnection.
For example, first established vpn has no DNS server, then switch to the one
with DNS server, the system continues to use the DNS servers from the default
network in addition to the ones assigned from the VPN server, resulting in a
DNS leak. In the reverse case it causes DNS resolution fails. New API
resolv_has_nameserver() get DNS information from the DNS resolver directly.

Bug: 116539103
Test: verify patch via unmerged CTS test case aosp/658122 (with
little modification)

Change-Id: Ie01814435f4361c258c4ba96a47eb917f5441274