Do not return to Home activity if not top task.

Add a test for being the top task when closing the last activity in a
task. Without this test, if the last activity in a task was flagged to
return to the home activity on completion but that activity had
launched a new task we would not go to the new task but would instead
launch the home activity.

Fixes bug 8775949.

Change-Id: I7fd9d9ebbdbff8064d894407c35190894a141ecf