Fix default network validation overcounting

When switching from a validated default network to a new validated
default network (typically because of a better score),
DefaultNetworkMetrics would not reset the last validation timestamp.

This would cause the new default network to have a total recorded
validation time overcounted by the validation time of the previous
default network.

The following fix should be applied downstream for consumers of
previously recorded data:

  validation_time = min validation_time, duration_time);

Test: runtest -x frameworks/base/tests/net/../IpConnectivityMetricsTest
Change-Id: I303d11023527c19435f5f5e796a0295ae3f76d9f