privacyguard: Share privacy package name between activity stacks

AOSP 4.4 now has a stack supervisor with differents activity stacks for home and focus activities.
Every ActivityStack instance has a ref to this supervisor. Just share the privacy guard package name
throught the supervisor.

Thanks Jorgue....way better solution then the dumb fast one I did on 4.4 bringup

Change-Id: I66c283fb9e3cec6ac9ec6a293e068d115e80a3fa