Add slight delay to touch outside for multiwindow

Similar to ag/28169421, add a delay so that when a back gesture swipe
detected we don't rapidly change focus to freeform window at the back.

Fix: 359349899
Test: Presubmit and manual
Flag: EXEMPT Bugfix
Change-Id: I1ef354b59a1a2e31dd36fa0171a3806a36ea21df