NetworkMonitor: send one DNS probe per web probe

This patch changes sligthly the two web probes mechanism for captive
portal detection and network validation so that DNS resolution is always
done for both probes.

In general the target web servers of the two parallel HTTP and HTTPS probes
are now different. This introduces a bias in the latency measurement of
th HTTPS probe since this latency will also include DNS resolution in
general.

Test: manual verification + $ runtest frameworks-net
Bug: 32198726
Change-Id: I71ed6927f5d9cdab0d0236cc147fe1c5735a1245