Split network monitoring and portal probe events

This patches changes how captive portal tests and network lifecycle
events are logged as connectivity events:
  - it splits NetworkMonitorEvent into two event classes:
      - ValidationProbeEvent for logging individual probe events.
      - NetworkEvent for logging network connection, validation,
        lingering, and disconnection.
  - it removes the redundant CaptivePortalCheckResultEvent class.
    The information logged in CaptivePortalCheckResultEvent was already
    logged by NetworkMonitorEvent, but missing the evaluation durations.
    It is now logged by ValidationProbeEvent.
  - it removes the CaptivePortalStateChangeEvent class, which is now
    redundant with NetworkEvent, but missing evaluation durations.

In addition, it adds event logging when ConnectivityService puts a
network into lingering or removes a network from lingering.

Bug: 28204408
Change-Id: I8f9752e4d36175ecfcbd1545a01a41bad6e06ea4