Update debug.tracing.desktop_mode_visible_tasks system property.

When the number of visible windows changes, output a system property
that reflects the count. This will be used to set the initial value
for the trace counter.

Bug: 358369221
Flag:EXEMPT, trivial change.
Test: atest DesktopModeLoggerTransitionObserverTest
Change-Id: I29cc8bbbaedaee0fb650400848b41b7a98d769f9