Re-factor state machine in NetworkMonitor

In ProbingState, most events are deferred and never exited if
the probe complete message indicated that the probe timed out or
failed. It means it will need to remember to explicitly return
HANDLED or NOT_HANDLED in the ProbingState for every new
message. Thus, re-factor the design for better architecture.

Test: - atest FrameworksNetTests
      - manually test for state transition
Bug: 120014928
Change-Id: I18500b958b35383335fcdef6af4e08dbbdfdffb0