Add unit test for NetworkStatsSubscriptionsMonitor

Bug: 154080205
Test: atest FrameworksNetTests:com.android.server.net.NetworkStatsSubscriptionsMonitorTest
(copy from ag/11788484 but getActiveAndHiddenSubscriptionIdList() since it's renamed
in internal)
Change-Id: Ia162f36efe96d90b2f6f70190d9fa855bba34484
Merged-In: I4ef3f1ab2e5ea1322133edd80db3ccd0022e9c91
2 files changed