Centralize handleAppDied and fix return to home.

The home activity was being returned to when any activity in a task
that was launched from home crashed. If there were still activities
left in the task then the crash should have brought up those
activities next, not home.

This may be a partial fix for crashes where the back stack was showing
up under launcher icons. Bug 10858941.

Change-Id: I840a25bd8395bfce46f4e21b112d78b12884706d