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
Merged-In: I729bc9cd7ba6cfc088aaf0aec3e770f14d1ac10d
Merged-In: If62c3348fe115f9791a136bf16ccf8bacccff36e
Change-Id: I15a9c2d328fec2910e47a477cbc1dcaa5323675a