Do not treat the dummy animation as an animation

The dummy animation is a place holder. When the task stack is being
removed treating it as a true animation forced us through a path
where we deferred the detachment and made a call to tmpRemoveWindows.

Also replaced call to tmpRemoveWindows with a real remove windows
call. The tmpRemoveWindows call does not clean up the Session
and this leaves a surface on the screen.

Fixes bug 15591610.

Change-Id: I05f7bc276bfed2366bbcd8443c92a4bbbc8a4491