Fix race condition between binder deaths

If a Binder dies there is a race between activity manager and window
manager to see who can handle the binderDied call first. If the
activity manager wins the race it will remove the activity and task
but leave the windows around. Until the WindowState.binderDied call
is made and all animation is complete the windows will try to access
the task that they were associated with.

This fix removes the windows of an activity when the activity is
removed. It also defers removal of the activity and task until
exiting windows have completed their animation.

Fixes bug 17031518.

Change-Id: Idf52f55c5feb0cad4e3664ef2eae5b7e95bbf490