NetworkMonitor metrics: add first validation information

This patch adds first validation information to:
  - ValidationProbeEvent, by extending the probe_type int field of to
    also include a bit indicating if the probe was part of a first
    validation attempt or not.
  - NetworkMonitorEvent, by defining new contants for the event_type
    field.

Test: $ runtest frameworks-net
      + manually generating events and inspecting the
        output of $ adb shell dumpsys connmetrics list
Bug: b/32198726
Change-Id: If5687e310ddd18ed3096f8ecbdfc2ef866519234