Don't continuously reevaluate DNS-over-TLS unless strict mode

This is because sending TCP SYNs and not getting any replies can trip
the mobile data stall detector. The data stall detection mechanism
needs to be re-evaluated, but until such time only do reevaluation
if a network is using strict mode.

Additionally, revalidate failed servers whenever a DNS configuration is
pushed down to netd.

Test: as follows
    - built, flashed, booted
    - observed tcpdump traffic in opportunistic and strict mode
    - ./system/netd/tests/runtests.sh passes
Bug: 64133961
Bug: 72344805
Bug: 109928338
Change-Id: I15a9c2d328fec2910e47a477cbc1dcaa5323675a