[9/n] Add log for active and inactive desk

Before multiple desktops, desktop windowing session is active only if
there is at least one freeform window open. Now, desktop can be active
and empty. Log desk session enter and exit from DesksTransitionObserver.

Session exit will be marked as pending in DesksTransitionObserver so any
task removal changes are logged to the correct session. After any task
info changes are logged, session exit will be logged in
DesktopModeLoggerTransitionObserver.

Flag: EXEMPT logging
Fix: 423560267
Test: atest DesktopModeEventLoggerTest
      atest DesksTransitionObserverTest
Change-Id: Ica9a44d9b4be7b11d7dcfe4f3846c80c3750a071