Remove AppWindowTokens from exiting apps with task

When the task is removed from a task stack in window manager any
exiting activities left in the stack were orphaned. This led to a
memory leak. Removing all task activities from those that are exiting
fixes this problem.

Fixes bug 18943737.

Change-Id: I0a5ea8d2d3be89af7ccaf01385a226a2eafdf507