Avoid race condition in heads up snooze code path.

The user can still snooze the heads up as its in the process of timing
out.  In this case, mHeadsUp is already null.  We save the package
name so that we can still snooze the package.

Bug: 18639224
Change-Id: I9bcce99a5c912e70a6856b277b61adab9cdcbc33