Fix starting window isn't removed

In multi-window scenario, there's a case that the Activity that can't
be starting window target is the last Activity to draw the first window,
which blocks starting window removal. Also, when the Activity finally draws
its first window, it can't remove the starting window because it doesn't
conatain starting window information.

This CL changes to only verify activities which can be the starting
window target. That said, activities that can't be the starting window
won't block starting window removal.

Test: atest ActivityRecordTests
Test: manual - reproducible steps in bug
Bug: 228194878
Change-Id: Ibc004ca079f0b04609183cd3b6630d70662fdef5