Rename isCloseTransition to a specific name

Renaming DesktopTasksTransitionObserver#isCloseTransition to
shouldAddPendingTransition.
"isCloseTransition" sounds like it returns true to all close
transitions, but actually it returns false when there are no freeform
window remaining in the display and it exits desktop mode.
Bug: none
Test: m
Flag: EXEMPT refactor

Change-Id: Iacaf0c2be9b7eddc9e5ad32773fa64edb5e6ff74