Fix PDM_UNKNOWN in private_dns_modes

When a lookup can be found in the cache, the code doesn't go in
res_tls_send() and then misses to set private_dns_modes. To fix it,
set the value in the beginning of the lookup threads to ensure
it is set at least once.

Bug: 144881031
Test: m com.android.resolv
Change-Id: Id3a3383403a381c5f73f7214ce1d768a98bb2eb5