Properly abort queued transitions during clean-up

When cleaning-up/flushing transitions due to player
disconnect, it is possible for queued transitions to have
moved to collecting but still have posted the actual
work on the handler. If sysui dies/disconnects in between,
then we can accidentally run logic that assumes the
transition is still valid after it has been aborted.

So, add a check to skip this work if transition has been
aborted.

Bug: 439829314
Test: observe test metrics
Flag: EXEMPT bugfix
Change-Id: I5a7df2e891e4d8940cbcf16259506df6902bba66