Clear all lingering notifications when network is disconnected

In aosp/861571, it tried to refine the design to prevent the
lingering notification is cleared unexpectedly. However, all
notifications should be cleared when network is disconnected.
Refine the design by clearing all lingering notifications when
network is disconnected.

Bug: 123488640
Bug: 123537643
Test: 1.Build pass
      2.atest FrameworksNetTests
      3.Connect to a hotspot which doesn't have internet,
      after getting "NO_INTERNET" notification, disconnect
      the wifi and check if "NO_INTERNET" notification will
      be cleared or not.
Change-Id: Iac45ec186cf81915c318dba37f43136609d579f6