Injecting network validation stats into statsd

1. Fill in each field of the NetworkValidationReported
2. Write the NetworkValidationReported into statsd

This patch also refactors tryCapportApiProbe to return null when the
capport data is incorrect, instead of doing the check after calling the
method. This makes it easier to compile capport API probe metrics.

Test: atest NetworkStackIntegrationTests NetworkStackTests
Test: atest FrameworksNetTests
Test: Manual test with statsd_testdrive
Bug: 151796056
Original-Change: https://android-review.googlesource.com/1295496
Merged-In: Icf34402d6a293cc76c32d00835cbf358c99a87fa
Change-Id: Icf34402d6a293cc76c32d00835cbf358c99a87fa

• apishim/30/com/android/networkstack/apishim/api30/CaptivePortalDataShimImpl.java[diff]
• apishim/common/com/android/networkstack/apishim/common/CaptivePortalDataShim.java[diff]
• src/com/android/networkstack/metrics/NetworkValidationMetrics.java[Added - diff]
• src/com/android/server/connectivity/NetworkMonitor.java[diff]
• tests/unit/src/com/android/networkstack/metrics/NetworkValidationMetricsTest.java[Added - diff]