Do not delete stack if empty task is being reused.

If a task is being cleared due to Intent flags and that clearing
was causing the last activity in the last task on a stack to be
deleted, then we were deleting the stack. Immediately after
this we were refilling the task with the same activity but the
stack had already been deleted.

This fix preserves the task and stack when it is only being
cleared for the new activity.

Fixes bug 15611901.

Change-Id: I7bcfa826d0852d9c640daa73fe1324b89471fecd