Make private DNS connect timeout configurable

It could take time to connect to a private DNS server if the system
allows 6 syn-retransmissions (net.ipv4.tcp_syn_retries = 6), which
can take time more than 2 minutes.

This change allows us to configure the timeout value via dnsresolver
binder service, and keep the default timeout value the same as the
original design.

Bug: 120182528
Bug: 141218721
Test: atest --include-subdirs packages/modules/DnsResolver
Test: m com.android.resolv
      adb install com.android.resolv
      rebooted
Change-Id: I8711a31172cfc671bf348191db363e7863831470