Pipe onTaskVanished into DesktopModeLogger,,,

In some cases apps may get killed and we don't get a change for the task
being closed in that transition. This results in logging going out of
sync and also leaking taskInfo objects. This cl pipes onTaskVanished
into the observer to fix those cases.

Bug: 367713257
Test: Manually lmk apps, see that app vanishing is being logged
Flag: EXEMPT Bug fix
Change-Id: If4d813105e2c94ac90fd9de8ff5e2a4c9529ace4