Fix NPE in overridePlayingAppAnimations

It's not guaranteed that findMainWindow() is non-null, which can happen
if the window was just added, so we should not crash in that case.

Change-Id: I1f04876ef48bfc7636320124b7f60d6dfa0b24a2
Fixes: 31767124
(cherry picked from commit 6e5f3d2e)