Change task removal sequence

- Remove task after removing activity. Removing the task before
removing the activity led to problems when the task was still needed.

- In WindowManager wait for activity manager to remove the task.
Previously was automatically removing the task when the last activity
was removed.

Fixes bug 12957434.

Change-Id: I7f6b1f09a2e2a845f177f337769075c9376f8bdb