[DO NOT MERGE] Fixed NPE when trying to animate a window without display

In some cases it is possible for the AppToken.allAppWindows list to
get out of sync with the list of windows known to WMS if the client
doesn't call Session.remove(Window). This can lead to an NPE when
the animation threads runs and the display for the window has been
removed.

Bug: 19972099
Change-Id: Ifdf9ff2364b96757bba0539394c4a682f64577c9