This change includes:
1. Use service handler looper when initializing the observers
to simplify the structure.
2. Make the handler variable final in the observers class.
3. Remove misleading comments, where the callback events
are actually dispatched on the oneway binder thread,
which does not block handler thread.
4. Update tests accordingly.
Test: atest ConnectivityCoverageTests:android.net.connectivity.com.android.server.net.NetworkStatsServiceTest
Test: atest FrameworksNetIntegrationTests:com.android.server.net.integrationtests.NetworkStatsIntegrationTest
Test: atest CtsNetTestCases:NetworkStatsManagerTest
Bug: N/A
Change-Id: I5be92853b5bbc795a36bf774956bdac33d79099b
8b1d824ad2