InputDispatcher: Allow all windows to be removed from a display

Previously, if there were no windows reported on a displayId by the
OnWindowInfosChangedListener, InputDispatcher did not remove the
existing windows from that displayId.

Here, we fix the bug so that when OnWindowInfosChangedListener reports
no windows on a display, the windows are removed from the displayId in
dispatcher.

Bug: 239788987
Test: atest inputflinger_tests
Change-Id: I0b50d678982f9274d72a2e503fa65707b14b9ede