Fix bug removing all windows that belong to a display.

Removing one window causes its subwindows to also be removed.
We have to be careful when traversing the window list
because multiple windows may be removed at a time so we
don't get IndexOutOfBoundsException due to the window
list changing in unexpected ways.

Bug: 7273702
Change-Id: I0ed9ba00c325ad178ab28919ce2e763cb6fd38ba