Slightly delay touch outside event on embedded activity

So that we can drop the event if the back gesture swipe
detected and started in order to prevent rapidly changing
focus to the LHS activity and back to the RHS activity,
or the focus change back-n-forth could result in IME flicking.

Bug: 350493117
Test: wm presubmit
Flag: EXEMPT bugfix
Change-Id: I73928a666e7d794be851860058cee7936577378f