Address comments from aosp/1298476

This patch addresses the followings,
- Pass Looper to NetworkStatsSubscriptionsMonitor constructor
- Replace Looper with TestLooper in unit test
- Assert fail if result of condition check is not expected.

Bug: 154080205
Test: atest FrameworksNetTests:com.android.server.net.NetworkStatsServiceTest
      atest FrameworksNetTests:com.android.server.net.NetworkStatsSubscriptionsMonitorTest
Change-Id: Ibcaba2b38af80cc0ec9a4e428a3b3a1538bc4325