app_mode: report network connectivity drops.

Add NetworkConnectivityMetricsService to count and report network
connectivity drops during a kiosk session.

BUG=b:214364560
TEST=unit_tests --gtest_filter="NetworkConnectivityMetricsServiceTest*"
TEST=plugin/out ethernet cable during kiosk session several times \
     check kiosk-metrics.networks-drops in Local State \
     check Kiosk.Session.NetworkDrops after reboot.

Change-Id: Ib1d3f2337793a10a6fae98426ed73d708156e8f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3683164
Reviewed-by: Irina Fedorova <irfedorova@google.com>
Reviewed-by: Sam McNally <sammc@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Auto-Submit: Polina Bondarenko <pbond@chromium.org>
Commit-Queue: Polina Bondarenko <pbond@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1017120}