Preserve other windows during resize as well.

Before this change if user resizes a window that has a dialog activity
in the front, and a paused activity in its back, the paused activity
will be completely torn down and relaunched, causing a whole period of
black window showing to users.

Bug: 110716902
Test: The activity stopped flickering and go/wm-smoke.

Change-Id: I0dfe6a9c3d925a365a1a3beaa5841e1606220ac2