Test for task in front must include stack in front.

The CL that ensured that a dying task must be in front of the user
(ag/374996) only checked that the task was at the top of /a/ stack,
not on top of the frontmost stack. This checks the stack for being
frontmost before switching to home.

Fixes bug 11208762.

Change-Id: I43f6d380e7a880ec19db03711ada6c7437e15f73